That would be the worst plot armor and the worst thing to ever happen in OP, that's again just my opinion.
Here is the counter, why not do it to Luffy and luffy would become stronger than Kaido in (20 years)
Logically it should be he gains more strength because he is an adult but still has the same/very similar swordsmanship skills, etc
Why would his body develop but his skills won't?
If Momo ages 20 years and becomes a lanky adult with the strength of ur regular person that doesn't train (i.e. comparable to a Wano Merchant) then you'd have a point but if he gets more muscular or gain impressive physical strength which I think is very likely, then it'd mean that the effects of Shinobu's mature fruit also takes "training" into consideration as you wouldn't ordinarily gain an impressive build and strength by just aging
Also you have to remember that Oda has constantly drawn this Kid training and Tama even said he was very diligent with it, so I expect the aging process to put Momo were he'd have been in 20 years if he continued training. Personally I'd be very surprised if he has shit swordsmanship skills.
Lastly, it's important to note that when Kaido spoke about the scabbards not being able to open his scar, he said that their "swordmanship" pales in comparison to Oden's and we haven't really got a callback to that since then, even with Zoro on the roof, so chances are that we could get that with Momo ^^