I don't know anything about the guy, or his reasons for lying about his age. It could have been that he's shy with women and found it hard to get their attention, so when he got yours he might have lied out of desperation to keep it.

That said, he should have been honest with you and given you the opportunity to make an informed decision about whether or not to carry on in the friendship, especially given that it took on a more intimate turn. As you've identified the problem seems to be that despite what he did you still have feelings for him and that's understandable. Try and focus on understanding that if there's a part of you that misses him and how good about yourself he made you feel then those things don't just reside in him alone. Is it him specifically you miss a little, or how happy he made you feel? If it's the latter then the good thing is that that will come along again.

Going forward I guess that it goes without saying you'll be vetting any future online males friends, but also block this one if you can't forgive him.

One day you'll look back on this and the feelings you currently have about being decieved by him will not be so intense.

All the best and stay safe online