on 2019-10-31 14:21:06 UTC
But not impossible. Depends by early First Age whether you mean in its strict sense (after the Flight of the Noldor) or in the generic sense of "the oldest age of Middle-earth". In Beleriand, yes, but arguably only likely for members of the House of Finarfin in the earliest years (because they had greatest contact with the Sindar), or possibly Caranthir's people (via the Dwarves).
Tolkien provided the backstory behind three of his scripts (Tengwar, Cirth and one I initially hadn't heard of, Sarati, https://www.omniglot.com/conscripts/sarati.htm).
In-universe, Tengwar are the creation of Fëanor and were developed for writing with brush and ink, replacing the Sarati, which (if memory serves) were the first writing system created, by Rúmil. Tengwar were brought to Beleriand by the Noldor and largely supplanted the local writing system, at least by the Elves.
Cirth (runes) were developed in Beleriand by Daeron the minstrel of the Sindar, and were also adopted by the dwarves. They were developed for inscriptions graven into stone although also could be written with brush/pen (as in the Book of Mazarbul). In later ages, I believe the Cirth were chiefly used by the Dwarves and were considered by most humans/Hobbits to be Dwarven writing.
In short, any stone tablets would be most plausibly written in Cirth, which presupposes sufficient time/contact for the Noldor to learn them. Unless you want to invent a Valinorean system of runes that were abandoned by the Noldor after they encountered the Cirth? (Not implausible : presumably there was a desire for stone inscriptions in Valinor, and Tengwar would be very time consuming to cut into stone, what with all the curves and flourishes.