Silver Surfers...
I understand completely what you're saying about people entering the wrong information - but I think the point is being missed a little. So what if I entered my details as being born in 1900 - they can still work out what films I like, the books I like - and get a pretty good idea of my age depending on the average ages of the friends I have - unless a very high proportion of my friends/acquaintances are adopting the same '1900' attitude.
The fact is that there are networks/communities on facebook that share common interests... let's not get ageist now.. age has little to do with it.