Does it matter?
He obviously thinks it does because he's been telling me for the last 3 months that she's 18.
I've only met her briefly and she seems nice enough. Her parents seem very taken with him and they spend a lot of time at her house, so assuming they know his age presumably they don't take issue with it.
They've been away for the weekend and her parents helped organise it so they don't seem to have any objections.
I found out because I was booking a thing for us all to do together and needed her DOB. DS1 was at work, so I messaged her and she sent it straight back, so I don't think the lie originates with her.
I'm really cross about the lying, but not sure if the actual fact makes a difference?