"prove the big mouse is an adult"
Let's see here assuming he's one guy and not a rotating cast of people to play the role (wouldn't put it past them tho)
Let's assume they were 13 when they started in 2018 or so
2018 was 8 years ago
13+8=21
Therefore, scientifically ironmouse is between 21 and 122 assuming he's one person and not a collective of multiple people.