The moderator of the first General Assembly of the Presbyterian Church in America was a ruling elder–the Hon. W. Jack Williamson. Since that time, the PCA has established a tradition of alternating between ruling elders and teaching elders in its nomination and election of moderators for the General Assembly. But this practice remains unusual among Presbyterian denominations. In the OPC, for instance, the moderator of their General Assembly is almost always a minister; only on a few occasions have they elected a ruling elder to serve as moderator, presumably as a way of conferring a special honor. And even within our own ecclesiastical heritage—looking back to the old Southern Presbyterian Church—it wasn’t always so, as Rev. R.C. Reed explains in this review of the PCUS General Assembly of 1914 :

“The Assembly elected a ruling elder to preside over its sessions. The law which makes the ruling elder eligible to the moderatorship of all our church courts is but a corollary of a fundamental principle of Presbyterianism–the parity in authority of all Presbyters. Our church did right to put this corollary into the form of law, and it ought not to suffer the law to lapse into a condition of innocuous desuetude. We cannot be accused of working it overtime. The law was enacted in 1886. It was seven years after that date before it received its first practical recognition in the election of Hon. J.W. Lapsley. Only four ruling elders have presided over our Assemblies in the twenty-eight years since the way was open for them to be honored with this responsibility. Always there is good material among the ministerial members to fill the office, as there was in the last Assembly, and there is never any reluctance on their part to serve, but they, as well as others, allow the propriety of occasionally electing a ruling elder in order to do justice to the principle of parity.”

[excerpted from “The General Assembly of 1914” by R.C. Reed, in Union Seminary Review 26.1 (October 1914): 4.]

This change to the constitution of the Presbyterian Church, U.S. was enacted in 1886, as Rev. Reed notes. The overture to enact this change first came from the Synod of Virginia and from the Presbytery of Chickasaw, in 1884. The Minutes of the 1885 Assembly (p. 432) note that:

The Committee on Bills and Overtures reported on the overtures from the Synod of Virginia and from the Presbytery of Chickasaw, which were sent to the last Assembly and referred by it to this (see Minutes of 1884, pages 249 and 250), asking an amendment of the Form of Government in reference to the duties of ruling elders when elected moderators of church courts. Pending the discussion, a substitute was offered by the Rev. P.T. Penick, which was adopted, and is as follows:

That the request contained in these overtures be granted and that the Assembly hereby recommends and sends down to the Presbyteries for their advice and consent thereunto the following:

That to the clause in the Form of Government, Chapter IV., Section 3, Paragraph 2, stating that ruling elders “possess the same authority in the courts of the Church as the ministers of the word,” shall be added this sentence, “When, however, a ruling elder is moderator of a Presbytery, Synod, or General Assembly, any official duty devolving upon him the performance of which requires the exercise of functions pertaining only to the teaching elder, shall be remitted by him for execution to such minister of the word, being a member of the court, as he may select.

In the PCA’s Book of Church Order, parity among elders is noted in BCO 8.9 :

Elders being of one class of office, ruling elders possess the same authority and eligibility to office in the courts of the Church as teaching elders. They should, moreover, cultivate zealously their own aptness to teach the Bible and should improve every opportunity of doing so.

Comparing that present text with an overview of how this paragraph has changed over the years. First, the PCA text as it currently reads:

Elders being of one class of office, ruling elders possess the same authority and eligibility to office in the courts of the Church as teaching elders. They should, moreover, cultivate zealously their own aptness to teach the Bible and should improve every opportunity of doing so.

Then the text we had in 1973 and where that text came from out of the Presbyterian Church, US (aka Southern), dating back to 1888:

These Ruling Elders possess the same authority and eligibility to office in the courts of the Church as the Ministers of the Word. They should, moreover, cultivate zealously their aptness to teach the Bible and should improve every opportunity of doing so, to the end that destitute places, mission points, and churches without Pastors may be supplied with religious services.
1. PCA 1973, 9-2, Adopted text, M1GA, Appendix, p. 131
2. Continuing Presbyterian Church 1973, 9-2, Proposed text, p. 9
3. PCUS 1933, X-§41
4. PCUS 1925, X-§41
and
5. PCUS 1888 (cf. PCUS Minutes, p. 424)

These Ruling Elders do not labour in the Word and doctrine, but possess the same authority in the courts of the Church as the Ministers of the Word.
[PCUS 1879, IV-3-2]

These Presbyters, as ecclesiastical rulers, possess the same authority with the Teaching Elder.
[PCUS 1869 draft, IV-3-2]

These presbyters, as ecclesiastical rulers, are of the same rank, and possess the same authority with the teaching elder. And while the titles of bishop, pastor, and minister, belong to the teaching elder by way of eminency, because he excels by reason of his entire consecration to the work, as well as by the superiority of his functions, they also belong to the office of the ruling elder, seeing that, in order to rule with diligence, he must take the oversight of the flock; in order to its protection he must guard and guide it; and in order to discharge the chief duty of his office, he must serve Christ diligently in the exercise of government.
[PCUS 1867 draft, IV-3-2]

It becomes clear that the provision or recognition for having ruling elders serve as moderators of the higher courts was something which was from the start embedded in the Book of Church Order, even though that field of service was not always recognized or practiced by the Church.

And here concluding, the commentary of F.P. Ramsay (1898), though written in reference to the PCUS BCO, still pertains :

43.–II. These Ruling Elders do not labour in the Word and doctrine, but possess the same authority in the courts of the Church as the Ministers of the Word.

officially (for nothing is here decided as to what others than Ministers of the Word may do unofficially in the Word and doctrine),

but possess he same authority in the courts of the Church as the Ministers of the Word.

May he then be Moderator of a court, and of the higher courts as well as of a Session, seeing that to Moderators are assigned certain duties that only Ministers can perform?
Yes.

When, however, a Ruling Elder is Moderator of a Presbytery, Synod, or General Assembly, any official duty devolving on him, the performance of which requires the exercise of functions pertaining only to the teaching Elder, shall be remitted by him for execution to such Minister of the Word, being a member of the court, as he may select.

The Minister must be a member of the same court, so that he may be under the control of the court. It is to be observed that by a court consisting of the Word, men may be appointed to ministerial functions, and are subject to the control of the court, the power of government extending over the Church and its officers in all their functions. It is also to be observed that the Moderator is appointed to a special work by a court, and is answerable to the court appointing him. It is further to be observed that there is no fundamental principle requiring that the Moderator shall be of this or that class of Elders; but, since, as a matter of conveniency and prudence, certain ministerial functions are, in the detailed regulations of the Form of Government, assigned to the Moderator, the principles of the system do require either that these regulations should be abolished, or that Ruling Elders be kept out of the position of Moderator, or that a special provision, such as this, determine the assignment of ministerial functions. Provision is made elsewhere as to the Moderator of the Session.
[F.P. Ramsay, Exposition of the Book of Church Order(1898, p. 55-56), on IV-3-2 :]

Words to Live By:
All in all, prayer not just for your pastors, but for all the elders in your church. Pray that the Lord would enable and sustain them in their duties on behalf of your congregation. And encourage them in their work when you can.