Why can’t people act their age?

I watch a lot of TV shows and the one thing that I’ve noticed is that the actors never seem to be acting their age literally. Take for instance, the show F.R.I.E.N.D.S. (one of my all time favourite by the way), the cast was 2 to 5 years older than the age they actually portrayed. The popular show Vampire Diaries portrays teenagers at the age of 17 or 18, who are actually 25 to 35 in real life. The main character from Covert Affairs was said to be 28 when she started out, when in reality she was 34.

May be Hollywood actors always seem to think that they look younger than they really are. Or may be, younger actors can’t portray the roles all that well. Which makes me wonder if people of those ages should be doing the things that they are doing on TV, like on Vampire Diaries.

One of the few shows where the actors actually portrayed their age is Sex and the City, where the main cast all played their age. I’m not sure about all the other characters on the show though.