Guys this is certainly one of the least worthy things of stressing about.
I turn 26 in a month. That's closer to 30 than 20.
I think there are 2 reasons why people get worried about their age. The first is the fear that people's expectations of them will be different - certain types of behaviour are ok for a 17 year old but not for a 23 year old etc, you are expected to have 'achieved' certain things as you get older. The second is the fear of physical decline (eg looking older).
Now on the expectations front - plenty of people my age have got married, have a house, a car, and having graduated 4 years or so ago they are already starting to earn good money and on the way to a good career. This is the 'expectation' of a 25 year old.
I'm single, I don't have a driving licence let alone a car, and I'm only in my 2nd year at uni. I don't have any inferiority or jealousy towards them, because when I was 18 I worked briefly with a 25 year old who was doing temp work while being a student at uni, and he was the 'coolest' person I knew, he was out having a ball when his mates were all with their noses to the grindstone of their careers. Whether it was subconsiously or not, I have become him, seven years on.
Some people cage themselves into thinking there's certain patterns of behaviour that are ok when you are a certain age but not at others. Going out at the weekend with your mates, playing on the playstation with a few cans of beer and a pizza, that's ok when you're 18-21 but 25...oh no no. I find that form of thinking restrictive and it's entirely in your own head - I do the same stuff now as I did back then. Half of the reason people stress is because they think they have to abandon the things they enjoy doing because they are "too old for it", I live with a lad who's 23 and decided this year that he wouldn't come out with me on indie-nights any more because he thinks he's "too old for it". His loss!
Now, on the second point, the stress about looking older - I can speak from experience here - it does pay to look after yourself and your health. Most people my age look a lot older than me and one of the reasons I don't get upset about ageing is most people think I'm a few years younger than I am. I reckon a lot of this is to do with the fact I didn't go to uni at 18 and so didn't batter my body with cigarettes, alcohol and late nights....these have an effect on your looks younger than you think because some of the people I envied at that age when they were at uni and I wasn't have very quickly started to get the skin leathering that happens when you get older, particularly the ones who went most wild. I've always eaten well and I keep myself fit and it has kept me looking young. So be wary of that one if you're the type to say "who cares if I die young, at least I'll enjoy my life" - you may or may not die young but if you're abusing your body you will start to look older by your mid twenties.
There is nothing to fear about getting older because you will gain something which is difficult to describe but comes under the ambiguous heading of 'life experience' which is often used to patronise younger people. Life experience calms you down, you get more used to dealing with situations and getting out of them and things that used to panic you or work you up don't any more. Also it gives you a sense of self and accepting yourself, you are less worried about what other people think of you and having to move into the crowd.
You get respected when you get older, people find it harder to put you down. That's why I'm a big advocate for people who haven't been to uni and think its passed them by, to go in their mid 20s like I did - you get to enjoy the social and academic experience without having the stresses and worries that affect you when you're 18.
I'm the same age as Michael Owen and when I was doing my A levels he was scoring that great goal for England against Argentina in the World Cup, and the commentators always said "he should be doing his A levels but he's out here in France...."
I haven't heard anybody yet say Owen is too old or past it.
Perhaps in a few years when he signs for Bolton or Man City on a free transfer and people start saying Owen is ageing and past his best, thats when I'll start thinking about getting worked up about my age.
But don't worry about getting older, all the things that trouble you about it are entirely in your own head and you can get rid of them yourself. The only things I would say in my old and paternalistic position are don't become boring as you get older, and make sure you keep an eye on how you're treating your body because you will appreciate that much sooner than you think.