Est Question

She looks like a young adult to me? She gets married to Abel in the original game and they are both adorable :)))

Latent dragon genes? She's not the loli-"Est" looking girl among the group

The artist who did Est's art tends to draw women looking *really* young. Compare Lachesis's art to her more age-appropriate voice. Mist is also only a couple years younger than Ike, but man you wouldn't figure that from her art.

Est is actually 16/17 and Abel.. idk probably over 18 so take it as you will. It's Japan, after all

The game is supposed to portray a cultural equivalent to the middle ages, with the obvious poetic freedoms to create a fantastic story for today standards.In any case, if beyond the fantasy on battle their timeline shares customs about marriage and relationships with the middle ages, it would be normal for a underage girl to marry/be coupled with a much older man.
Or scratch that, anime fans (the target audience for FEH) like their waifus young. There's no need for further explanation.

Same with Catria who I believe is the middle sibling. She's got a very serious and mature voice but looks like a teasing loli in her Heroes art.