L (sniff sniff) has been 25 since 2004, but if Kira hadn't viciously taken his life, he would be 30. Near is 22, (17 when the book ended, I think) Mat and Mello, well I'm not so sure about them. I think they may be the same age as Near. Kira died at about 23. (ha ha, Kira! You deserve such an embarressing, painfully slow death for what you did to L!)
L was born 31st October 1979
Mello on 13rd Dec 1989
Near on 24th August 1991 (my same day!!)
Matt on 1st Feb 1990
Raito on 28th Feb 1986
Misa on 25th Dec 1984