whats the oldest uve fancied of the opposite sex?

I wouldn't admit that. When you meet a girl, in a club for example, the biggest tell-tale sign is when you have a conversation with them, much more than the way they look. Some girls are more mature than others but there's always a certain naivety/innocence that shows through. Some girls can fool you into thinking they're 2 or 3 years older but after a while, you still end up noticing.

thats a difficult topic cause i got to know this girl from judo she looked about 11/12 ish so i obviously didn't speak to her too much but when i found out she was 20 i couldn't believe it i was totally shocked!!! i mean everyone else said they thought the same at first but even if i found her attractive i still dont think id go out with her (if a rare instance came where she asked me) cause id feel way too old

is this just me or would other guys think the same

If you think the girl's cute, go for it.
Some people are bad at telling age. To me, even 18-year old girls that look 25, still look 18 to me. There's always something there. Not necessarily in physical features but in the way they hold themselves, facial expressions, stuff like that. But like anyone, there are times when you make errors of judgement.

if shes 20 how could you feel too old?

well id look too old and she only acted young as well, i mean the same confidence/dress sense as someone a lot younger.

if i liked her i wouldnt be worrying about dress sense to be honest

Maybe you're going to the wrong clubs

lol. Ok some girls can fool you a bit with the way the dress and the way they look. But there are so many things that can be tell-tale signs that a girl is younger than she might say.
For me, it's when you have a proper conversation with her. Depending on the subject, there are things you say that give away your age.