To me 'underage sex' is where both characters are underage for the country/state/area in which they live. For example if two 15 year olds had sex in the UK that's underage, but if they were 16 then it's not underage, because our age of consent is 16.
'Chan' to my mind involves an adult having sex with someone under the age of consent.
In some ways one could argue there is maybe a difference between a 16 year old having sex with a 15 (almost 16 year old) and a 30 year old having sex with a 14 year old.
And probably the former is nearer to 'underage sex' than 'chan', but then you get into all kinds of layers and levels as to what makes (a) underage but (b) chan.
I've seen it said that chan is when there is a large age difference an adult adult, not a teenager and the partner is either under 10 or a young teen. But like so many fandom terms things do seen to vary.