Someone in their 30’s will significantly change their worldview over the next 50+ years.

More than anything else, the church wants stability. If you elect someone in the last part of their life, you’ll have a much better idea of what ideology their entire papacy is going to be formed under.

If electing a pope was only for 4, 10, or even 20 years, it would be more reasonable to elect a younger pope, but as it is a lifetime appointment, someone older is always going to be a more conservative option.

Electing a young pope is too risky.
He doesn’t have a long track record and may still change his views, so he’s less predictable.
And he could guide the church in a “wrong” direction for half a century.

In the middle ages, popes were often elected at a younger age, but they were also less reluctant to “dispose of” a bad one, or excommunicate him for devil worship and elect a competing one.

You know how there’s a saying about banks,(I’m paraphrasing) “If you’re in debt $100k, the bank owns you. But if you’re in debt $1m, you own the bank”? Same way in church. If it’s one boy, or 100 boys.

They don’t want them in there long enough to make any real changes