Q. We know that Mary was a teenager (when she was betrothed), how old was Joseph?
A. Yes it’s likely that Mary was just a teenager, since that was the custom of the day. Joseph’s age is not indicated but circumstantial evidence can lead us to conclude that he was much older.
For example, the word generation is defined as the length of time from a man’s birth to the birth of his first child. In the Bible this averaged out to be about 40 years, suggesting that a man would normally be in his mid to late thirties at the time of his marriage.
Also, a Jewish man had to pay the girl’s father a negotiated “bride price” before taking her, and show that he could provide for her and the family they’d have with a stable income and a suitable house for them to live in. It would normally take years after learning a trade for a man to become financially prepared for marriage. So all things considered it seems likely that Joseph would have been much older than Mary.