I am one of the people that decided Ingo is old. I will try explaining why I thought so, and why this post doesn't disprove anything. All images are cropped from bulbapedia.
1. Those aren't sleep deprivation eye bags, they're age indicators
Here's Larry. Canonically, he's a middle aged man. He has grey in his hair, fat loss on his cheeks, and most importantly, permanent eye bags
Here's another character! He's also canonically a middle aged man. He also has fat loss on his cheeks, grey hair, creases on his forehead and permanent eye bags.
And lastly, here's Opal, to illustrate my point even further, namely — Pokémon designers use eye bags to show age.
I want you to compare the men that we saw before and Ingo, Again.
Those aren't eye bags. He looks similar to Larry in age, though not quite old as Opal and Kabu. That will change later.
Do you think Melli can't grow a beard. Be honest. Do you think he's naturally a twink. He's not. Sorry, anyway-
Do you think this is his natural growth pattern.
Something people tend to forget — Ingo's goatee is a choice. Keeping a goatee in this shape is a choice men make, and they save everything around their goatee, and they trim it to keep it in shape. Ingo actively chooses to keep this thing, like it or not.
This is Ingo without a hat. He wasn't modelled without a hat, but the official art book still has this.
Ingo has not only permanent eyebags, as discussed above, but forehead creases. See what Kabu has? Ingo has the weaker version of it. Not to mention his receded hairline.
In conclusion, Ingo isn't sixty, sure, but he's not young. Ingo is a middle aged man, and it's been a long time since he landed in Hisui.