Is there a comprehensive summary of Solidus' activity in his earlier years? Metalgear wiki only mentions that he's in "paramilitaries of the CIA", then dispatched to Liberia in the late 80's.
If Solidus was sent on missions during the 80's, before the Liberian Civil War, there are definitely two grown men that look pretty much identical to each other, with questionable differences in age. But then again, he might've been kept in very secretive operations etc. so perhaps he wouldn't surface till the end of the 80's.
It's interesting that in Liberia, Solidus adopts Raiden into his Small Boy Unit, and there could be a symbolic parallel between that and Big Boss and Eli (if Eli is Liquid) in MGS5.
In any case, Solidus is most certainly a grown man by 1984 (but a teenager at heart
) so we might see him anyway in MGS5, voiced by David Hayter, no less, hurr. But honestly, Solidus is just such a patched up attempt at a character, the whole US president stuff...