Where's your proof?
It's a rhetorical question, because there is no proof. We simple don't know and can only speculate.
Because we know historically Jєωιѕн women got married as young as 12, and the other ancient civilizations it was around 15. Since judae was under Roman occupation it makes sense she got married in her youth.
Even in the middle ages before protestantism most gis got married by 19.
It extremely unlikely and frankly foolish to believe that St Joseph was her age or even only a few years older as men take more time to develop, mature and become capable at doing their duties.
Since many Saints have given Saint Joseph a decent 'middle' age that would put the age gap at ~15 or so years.