Maybe not even venial. It depends on what the lie actually was.
If you are a minor and lied about your age to conceal that, maybe a venial sin.
If you are middle-aged person who says you are 16 in order to lure teens into conversation with you, maybe a mortal sin.
On the other hand there are good reasons to not be too specific about information shared online. I have a real-life friend who posts here. She is on her second user name. Under the first name, she was very honest about her birthdate and location where she lived. Someone figured out who she was and posted some personal information about her kids that she had never put on CAF. That really scared her. CAF doesn’t let you change your name so she re-registered with a new name. The second time around she did not list her real birthdate or her real city. In that situation, I don’t think it was a sin at all.