[Rivière]

Firstly, have you actually asked them why they lied about their age? Are you completely sure that the age they posted on another site isn't the age they've lied about and were truthful with you the whole time? Can you confirm which is the lie and which is really the truth?

I think rather than making assumptions, actually spend time with some probing questions to find out more before jumping the gun and making the assumption they've lied to you without proper evidence. The fact you're ready to drop them as your friend without actually giving them the chance to explain themselves is a little saddening. You may be surprised as to what their response is, if you give them the chance.

As you've said, they've never done anything inappropriate with you. You've developed a good 3-year friendship. Why throw it all away based on insufficient evidence and assumptions?

Talk to your friend and find out what's going on.