Used to be the WT said Timothy was maybe 30 and still considered young. This last study has him a snot nosed (my take) teenager, who is moving up. The actual Bible account only tells him being raised as a believer. None of the drama stuff about having a materialistic father (if he was even around), skipping "college", having worldly friends trying to influence him.
My own opinion is he was in his 30's, that is how long it takes to get a handle on life. But never let facts get in the way of an argument.