I also think it is very hard to guess what Rashid's real age is, remember he spends 90% or more of his time at the Gates in the Nevernever, time works differently there ...
That's my point, yes:
I believe Rashid is the oldest WC wizard, and I suspect that comes from spending so long in extreme time-dilation at the Outer Gates & other deep-Nevernever regions.
I don't know that he spends "90%" (or any other specific number) of his time at the Outer Gates (afaik this isn't something that's ever been specified), but it's certainly a
lot of his time... enough to hugely-extend the elapsed mortal-world time since his birth, without actually "aging" him.
But Rashid is
at least 1300-ish years old, in terms of mortal-world-years-since-birth, per WoJ. And I
think (I'd have to go delving to be sure) we also have WoJ that wizards' lifespan is normally 400ish years. That's a pretty notable discrepancy, whether taken as absolute years or as a multiplier.
... because he has spent most of those years in the Nevernever, his age appears to be or close to Eb's age..
I think wizards go through a huge long plateau of general good health & fitness with
very slow "apparent" aging, that begins in their mid/late 20's and lasts for around 300-350 years; then, when they finally begin to look "old" (apparent age 65ish?) they begin to age less-slowly (though still not fast) for another 50ish years.
AFAIK, there is no WoJ as to whether some kinds of magic (e.g. water-magic, which is related to healing), or an individual wizard's particular power (e.g. Harry is very-strong) get more of this benefit (whether "younger looks" or more actual years, etc) than others do.
I strongly suspect that two wizards' relative "apparent ages" are a very-poor way to figure out which is actually the elder of the two.