As far as I know there's no source for his age. Curious to see what everyone else thinks!
I'm glad we're all (mostly) in agreement hehe. I've always headcanoned Giovanni to be a good 10-15 years older than Jessie (so like 35-40). I love the idea that when his mom (Madame Boss) and Jessie's mom (Miyamoto) were in meetings, he was on babysitting duty. Big brother Gio