We're not comparing Dumbledore in his thirties to Snape in his thirties, but Snape in his thirties to AD at "64".

Right; by 'extra decade' I meant another decade above the three Dumbledore was already older than Snape.

It seems unlikely to me that what made Dumbledore able to defeat Grindelwald at ~78 was that he was 78, rather than that he was Dumbledore. From EY several comments above:

Bones, in this fic, is an ancient and experienced witch. Forty-four simultaneous strikes from upper-year Hogwarts students will definitely bring down Madam Bones, maybe even if she does have time to reinforce and strengthen her shields. Dumbledore would wave the Elder Wand, once.

We do know now that Dumbledore had a phoenix to help him out in the duel with Grindelwald, and so we might suppose that the equipment advantage was balanced between the two instead of heavily favoring Grindelwald. (My impression is that the Elder Wand is better to have than a phoenix in a duel, but that's just an impression.)

But I don't know what Dumbledore looked like in his early thirties. We see him at 18 and then at 64, and perhaps he passed through where Snape is now at about thirty. My guess is not.