According to the Medieval Lands site and Alison Weir's book Britain's Royal Families, Geoffrey was born on 24 August 1113, which means he was not quite 15 when he married. I'm not sure where I read it, but I have a feeling that Matilda wasn't happy about being forced to marry him, and the marriage wasn't consummated for a while for this reason, not necessarily because Geoffrey was too young.
In the meantime it is highly possible for a 16 or 17 year old to father an illegitimate son if Hamelin was born in 1129 or 1130.
I hadn't looked before but it's in Alison Weir's book, that Hamelin's mother was Adelaide of Angers, but as she unfortunately doesn't name her sources, and everything else I looked at sticks with his mother as being unknown, I think that is still the best answer.
John