Bravely Default 1 (Tiz at 19) & 2 (Seth at 21). BUT, 1 is a weird case in that every character was aged up by three years in the western versions, mostly due to certain costumes the party members can wear. Sooooo that one is debatable.

Dawn of Sorrow Soma is 19
Koudelka is 19 from Koudelka
Alphen from Tales of Arise im pretty sure is in his 20s
Robin from Fire Emblem Awakening is 19
Cross not sure on his age, but could be 20s. Elma could fit from Xenoblade Chronicles X. She seems to be over 20.
Roland from Ni No Kuni 2. Although he's a deuteragonist - Not sure if that would fit. Evan isn't really over 20
Really not sure on Radiant Dawn either. Can't find any info.
Stocke - Radiant Historia is 19
Xenoblade Chronicles 2 Torna - Lora is 27
Not sure if Triangle Strategy Serenoa would work. But they have equal protags and one of them is an old dude so that could work.
I'm also sure a lot of protags for front mission games could fit.

@AlphaOne2 - Bravely Default 2 added. I'm going with the original ages for BD1 so that's excluded.

@Detectivefail - Tales of Arise added. Going with Elma for XBX and she is apparently 29 so added. Added NNK2 for Roland. XB2 Torna added. Triangle Strategy is iffy, but I'll add it.

I just noticed shouldn't Sky the 3rd be here. Kevin was like 21-22 then I think.

oo this sounds fun

Ashley from Vagrant Story (late 20s)
[redacted] from Ys Origin (obvious)
The Batter from OFF (implied)
Garrison, Red Monika, and Knolan from Battle Chasers: Nightwar
Stick RPG (biggest jrpg stretch but if yakuza 1-6 is here fuck it. anyway ur a CEO, homeowner, drink in bars etc)
speaking of--Takayuki Yagami from Judgment and Lost Judgment should be here if kiryu is
Masaharu Kaito from Lost Judgment: The Kaito Files (obvious)
MC from SMT Strange Journey (obvious)
Kyouji from SMT Devil Summoner (199x, meaning he's between 22-32yo)
Lacryma Christi from Kartia (implied)
Zetta from Makai Kingdom (implied/obvious)
MC from Citizens of Earth (obvious)
MC from Citizens of Space (obvious)
Matthew Farrell from Penny Blood (obvious)


(on second thought, this was way harder than it had any right being--to the point of being a bit depressing..)

@SeymourFlux - Sky 3rd added. Somehow skipped my mind when I was adding the Trails games.

@Rola - Vagrant Story added.
I don't think there's any confirmation on the Ys Origin MC ages and it wouldn't surprise me if they were still in their teens so I won't be adding that.
Off added.
Battle Chasers: Nightwar added.
Stick RPG doesn't resemble a "JRPG" based on what I've seen so I won't be adding that.
I originally didn't include Judgment and Lost Judgment because they didn't have many RPG mechanics even when compared to the Yakuza brawler games. That said, I revisited the idea and I'll just include them. Kaito Files is DLC so I won't be adding that as a standalone game.
SMT Strange Journey added.
SMT Devil Summoner added.
I couldn't get much information on Lacryma's age from Kartia other than a Gamefaqs guide that says she's 18.
Makai Kingdom added.
Citizens of Earth added.
Citizens of Space added.
I'm going to hold off on adding unreleased games so Penny Blood is out for now.

Fire Emblem: Genealogy of the Holy War with Sigurd more than likely being 20+.
Live a Live has multiple protagonists, so the definitely 20+ protags are Sundown Kid and the Master, with Oboro and Oersted being probably 20+.
Ash from Voice of Cards: The Island Dragon Roars is a hard maybe.
YIIK: A Postmodern RPG.
Both Fear and Hunger 1 and 2 have a few 20+ year old protagonists.

@PunnyPeace - Added Fire Emblem: Genealogy of the Holy War.
Added Live a Live.
Couldn't find any concrete info on the MC for Voice of Cards: The Island Dragon Roars and doesn't seem to be at least 20.
Added YIIK: A Postmodern RPG.
Added Fear and Hunger 1 and 2.