The OED glosses geezer as "A term of derision applied esp. to men, usu. but not necessarily elderly; a chap, fellow. " Its first citation is from 1885. Of the ten citations, four (including the first) explicitly say "old" in association with geezer, and I think that all are British sources. One of the citations is "1893 Northumbld. Gloss., Geezer, a mummer; and hence any grotesque or queer character. " This suggests the equation grotesque = old as the source of the association with old age. In (my intuitions about) American usage, this association has become part of the core meaning of the word, and to use geezer for a child or youth would have to be a joke or other special effect.]
I understand what you’re trying to say acmcc but I don’t believe he meant it as “great bloke”, either way it doesn’t matter.