The Gospels don't say how old he was, and we don't have any historical sources about him from the time when he would have been alive.

On my phone so this will be brief: the main ancient document about this marriage is the "Gospel of James." This was influential in advancing the idea of Mary's perpetual virginity and that Jesus's brothers were actually stepbrothers. It doesn't say how old Joseph was, just that he was a widower and had kids. It says Mary was 12 at the annunciation. Note that I'm not saying this is a correct account of their relationship, only that this is the primary source for early Christian tradition about the details of their relationship. It was written about a century after Jesus was killed.

See http://www.earlychristianwritings.com/text/infancyjames-roberts.html