In old movies or vintage photographs, the cassock worn by altar servers was generally equipped with the classic white Roman collar. Unfortunately, I have noticed that in today's Tridentine Masses, altar servers are sometimes without it. Some have wondered whether the use of the Roman collar is reserved exclusively for priests. Years ago, during the pontificate of Benedict XVI, Daniele Di Sorco answered this question on a forum. Here are some excerpts from his interesting contribution:
There are no Holy See documents regulating the use of the Roman collar. [...] This abuse [of having reduced the collar to the sole distinctive badge of the priesthood, Editor's Note] has probably generated the belief that the collar in particular, and not the cassock (or—by way of exception—the clergyman suit) in general, is the badge of the cleric. Hence the difficulty for some to admit the use of the collar by laymen who, during liturgical functions, wear the cassock.
How can one respond to such a difficulty? In three ways: by highlighting the true function of the collar, by analyzing the ancient practice of the Church, and by examining the manuals of ecclesiastical etiquette that deal with the matter.
First of all, we must consider that the collar is a necessary complement to the cassock, from which it is separated solely for reasons of practicality (like cufflinks from a shirt or socks from shoes). Without a Roman collar, the neck of the cassock appears conspicuously incomplete: too low, too wide, inexplicably deformed by the square opening that reveals bare skin or the (often multicolored) collar of the garment underneath. It is precisely the unique design of the cassock that makes us understand how the collar is an integral part of it, and not a mere accessory (unlike, for example, the fascia: though in the case of the Ambrosian cassock, which cannot stay closed without a fascia, it is as indispensable as the collar).
Further proof comes from the fact that ecclesiastical documents speak of the cassock without ever mentioning the collar: a sign that the latter was considered an indispensable complement to the former [...]. But if from these and other documents it can rightly be deduced that the habitus ecclesiasticus or clericalis spoken of in the 1917 Code of Canon Law (can. 136, § 1) is the cassock complete with its collar, it follows that laymen, in cases where the Code allows them to wear the same habitus clericalis (can. 683), can and must use it. The text, in fact, makes no distinction whatsoever between the two garments. That laymen do not have the right to wear honorary insignia is, of course, a given, as these are strictly reserved for the clergy. The new Code of Canon Law has left this discipline unchanged, neither restricting nor modifying the use of the collar.
What emerges from the examination of canon law is confirmed by the practice in force prior to the liturgical reform. In many period photographs, it is clearly visible that the cassock worn by altar servers is equipped with a collar exactly like that of clerics. Where it was not used, this was due to the difficulty of obtaining a substantial number of collars in different sizes, not from the belief that it was reserved for the clergy.
The manuals of ecclesiastical etiquette that I have consulted, which are usually very accurate in determining which categories of people may or may not use certain insignia, never speak of the collar as a garment reserved for the clergy or a distinctive mark of the clerical state. If they do not explicitly mention the possibility for laymen to use it, it is because they too consider it a normal complement to the cassock. J. A. Nainfa (Custom of Prelates of the Catholic Church, Baltimore 1926), who takes care to specify that altar servers cannot use the fascia (to which, after all, not even all priests are entitled), makes no such distinction regarding the collar, neither in the chapter dedicated to it nor elsewhere. The same can be said of J. Nabuco (Ius pontificalium, Parisiis-Tornaci-Romae 1956).
Ultimately, it can be said that the collar is certainly a distinctive sign of the clerical state, but no more so than the cassock itself. Therefore, if the altar server, who during liturgical functions fulfills the role of a cleric (specifically that of the acolyte), is allowed to wear the cassock, there is no reason why he should abstain from wearing the collar.