One of my best friends is gay, came out when he was 19, but it had been pretty obvious since he was about... 9 or 10 I'd say. We had all been telling him that we knew he was gay from about the age of 15, and when he did come out it was probably the least shocking revelation in history.
On the other hand - there was a guy from the US in my University halls in first year on an exchange year, recently seen posts from him about hot guys / boyfriends / how being gay means you avoid the hassles of straight relationships (haha). None of us from Uni had any idea at all. I don't know if he was already out "at home" that year and for whatever reason decided not to tell us, or if he only came out later. But yeah, there was nothing "stereotypically gay" about him at all, other than the fact that he made a lot of female friends and yet always insisted that nothing was going on with any of them when asked... now we know why that was true
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So yeah. I think it depends on the person. Some gay men have obvious effeminate qualities and I think when that's the case, those tend to be quite obvious from a young age. Others are "manly" but just like men, and then it's not going to be obvious at all.