They know the phone is 3 months old by looking at the date of purchase, how long the sim has been activated for, and the IMEI Number. There are probably more ways too.
Orange will only unlock a payg phone if an orange sim has been activated 3 months and is registered to the phone. Not sure about contract but seeing as you will be most likely using the orange sim then regardless then it is much of a muchness.
Example if i bought this phone tomorrow but didnt regsiter an orange sim to the phone until say 2 months later i would need to wait 5 months from purchase to get the phone unlocked. The age of the phone is irrelevant. The time an orange sim has been registered with the phone is the only thing that matters. all a bit pony if you ask me and that is why i havent bought one... well that and the fact the moto have released the razr i.