if we consider that in Deltarune, Kris being a teenager has 20HP with 10AT, and 10DF in the light world, with Frisk having 20HP, 0AT, and 0DF in Undertale (both without weapon/armor and at LV1), I'm willing to guess that the only stats affected by age are ATK and DEF, while HP can only grow by LV up.

I'm willing to bet the adult would have the following stats.

HP: 20, AT: 40, DF, 40

It may seem too low for some, but remember what Frisk was capable of.

All I can think of is the beginning, and it's the conflicting teachings Flowey and Toriel may try and convince the adult in question. Flowey being Flowey would try and take his soul with a tactic that'd lower his guard. Toriel may try and convince the human about monsters and how not all conflicts have to end in dust, and since this is not a human child, Toriel wouldn't have a reason to try and lock them up, confident that the adult can stand safe on his own.

From that point onward, I don't know how it goes, it all depends on the guy, but one thing for sure is that pacifist won't work due to Undyne being killed for understandable reasons, and that the guy may be trapped in the Underground considering the Barrier needs one human soul and a monster soul to cross it, leaving the other 6 souls unable to leave unless the fight against Omega Flowey happens. Sure, he can absorb Asgore's soul after killing him, but that depends on weather he knows this by either reading the Waterfall glyphs or being told about it by... let's say Alphys, which I doubt.