To answer your first question: When trying to learn to change chords, (or any other aspect of guitar playing for that matter), the important thing is to take it slow to begin with. "Muscle-memory" is key and by taking things at a slower pace you're able to train your fingers and engrain the movements into them.
One excercise I used when I was learning chords was to pick one (for example C Major) and just practise moving that chord shape up and down the neck. This doesn't sound particuarly musical, I'll give you that, but that's not its purpose. What it does do is strengthen your fingers and help engrain said "muscle-memory". I know you say you can already play the chords, but by doing with with any and every chord you can think it your fingers become more flexible and it becomes much easier to get them to go wherever you want to.
As for changing between chords, you'll find (or at least, I found) that some chords are easier to change between than others. Isolate the chords that you find particularly difficult and practise going from that chord to another and back again.
As for your other questions:
21 isn't too late to start learning at all. Peter Buck, (REM guitarist) didn't learn to play until he was into his twenties, (23 rings a bell, but I might be wrong with that) and he's done alright for himself!
As for how long you'll have to practise, that varies from person to person. Me personally, I'd say it probably took me about 4 months to get to a point where I could go from any chord to any other without missing a beat, but some will take far less and others will take far longer. It's important to practise it and to try and challenge yourself. I'm sure you'll have heard this before, but it is just a matter of practise and patience...it may take time, but it will come if you put the effort in.
As for me: I'm 20, I've been playing guitar for about 7 years or so and dabbling in every musical style I've come across trying to learn something new.
I came across this site several years ago and I honestly believe that without it I wouldn't be anywhere as capable as I am now, (although I'm nowhere near as good as I'd like to be). I'd highly recommend you browse GT's lessons and see what you can pick up.
Also, if you're interested in carrying your playing to new levels, I'd suggest you become a high-bandwidth user, ($5 per month, which work's out at £2.99 at the current exchange rate). There's been a lot of discussion about the fee, but the vast majority of those who have paid it, (if not all), are very satisified with the benefits it brings.
Anyway, enough from me. Look forward to seeing you around the forum - anything you're not sure of ask, everyone's friendly and more than willing to help out as best they can. Best of luck!
Additional:
(Christ! I can go on sometimes, can't I?! Didn't seem so long when I was writing it, honest!)
Don't worry too much about me, ignore me long enough and I'll go away.