I think it's all about getting ahead on developing an attack--tempo. What do you think?
Castling

I like to castle as late as I can. I do it to keep the tempo going. Sometimes I castle after move 20 or 30.

I think that castling early cand help with pawn pushes, since you can now use your rook to support them. But i do find that people who dont castle are the most aggressive

Many lines of openings I play do not require castling. In fact, it would be terrible to castle. So I'm forced to fight it out in the center--a semi-closed center, so getting at the King is not easy.
In general, however, castling is a good idea to get out of the center and concentrate on the middlegame after tucking the king away safely. Castling or not is entirely dependent on the position.

OK, well what specifically do you look at to see if you should castle? (eg many pawns in the center, lots of pieces developed ect)

It depends on playing style also. For example I might leave my king in the centre to provoke the opponent to commit to attack, leaving weaknesses to exploit. If you play tactically then the strategy may suit. I won't castle if I can find other ideas more to my liking. horses for courses really.

I generally try to get castled in the first 10-15 moves. I think the king is an easier target in the middle and having your rooks connected gets rid of a lot of back rank problems. They also tend to be more powerful when they work together. Then again I play the Ruy so castling early is part of it.

Castling is considered a “development” move…
One simple "rule", once the queens are off the board it is not so important anymore… However, if it helps put the rook on the right file, then it can still be useful.
Castling is usually a good option to have even if you don't use it.
Nothing is always true in chess.
Castling is usually a good idea, but like DrSpudnik mentioned, it can sometimes make it easier for your opponent.
Do you typically castle every game? If not what makes you decide not to?
There is a chess store that i frequent. It has a area where you can freely play games. The people who play there are usually rated 1700-2000. I often play them and they dont castle. I was taught to typically castle every game (preferably kingside). So i was just wondering why a person would want to keep his kind in the center.