Really depends on several factors. You have Oven and Daifuku who have 300 mil but Snack and Peros 600-700 mil. Even tho in a fight Peros might lose to Oven, his ability is piss weak against even thr slightest heat, so against someone that uses fire he would have a hard time really, but he's the eldest son also, being for so long could also add to his bounty.
Perospero is only 2 years older than Oven and Daifuku - he is 50 while they are 48 like Katakuri.
But snack is 30 and has 600 mil. Cracker and Smoothy are also younger than Perospero yet have higher bounties. I don't think age has anything to do with it.
Oda shouldn't have given Oven and Daifuku such low bounties in the first place, even below Pekoms and Tamago, while they are clearly above them strength and Rankwise.
And speculating if they rarely ever left WCI is nonsense. Since we will never find it out anyways, there shouldn't be any reason for us to speculate anything. Oda should have simply give them higher bounties.