I'm inclined to agree with Maple that the implication is that the pronoun after
as old as
stands for a subject of an unstated clause. Perhaps the "rule" I
made up earlier should be extended to cases where the "finite clause"
can be merely
implied, i.e., need not be stated fully.
On the other hand, a significant number of speakers tend to say
as old as [us / me / him ...], thus treating
as as a preposition, so maybe the "rule" can stand as it is.
CJ