I think Bae was 14 when he went away...
This was before Emma was even born, so that would mean Bae would be at the very least, in his 40s.
Then it's pretty clear (to me, at least) that Bae was already gone when Rumple was training Regina, back when Snow was still a child, which is at least 10 more years, so he's at the youngest, in his 50s.

This is one reason I don't think it makes much sense that Rumple even suspected that August was Bae.

Also... Bae was already gone when rumple met Jiminey and gave him the puppet potion, right? Back when Gepetto, the old man, was still a child.

Just another reason i believe Bae has to be in Neverland.
Otherwise, he'd be dead.

I mean, Rumple even trained Cora however long ago, and Bae was already gone then too?
I'm confusing myself like crazy here.

Is anyone else willing to share any ideas? I'm going to drive myself insane.

See I'm not really sure either, I'm guessing in his 40's simply because of Gold assuming he was August. But what you said seems to make sense too.

For the sake of the story think he will be in his 30, but the truth he will be more than 300 years, rumple already became the dark one for 300 years plus the curse, 28 years.

Think Narnia time. It could have been seconds between Bae arriving and Emma arriving. I'd say if that's the case Bae is at least 42.

I'm not sure. Time doesn't seem to work logically in this show (due to magic, and stuff), we don't know how time works in FTL in comparison to how it works in the real world (like Fan46 pointed out, Bae could have landed in the real world just seconds before Emma), and we don't know where Bae actually went, so he could have not aged normally.

I'm so glad this has been somewhat cleared up! It's been bugging me for months.

He's neil, so between 20 and 30's.