Sakura no Sekai - Vol. 1 Ch. 2 - First Love

[MangaDex]

 

[winterume]

dammit now shes pregnant accidentally next chapter with all the hand holding and sudden kiss!

now Makima will hunt her down sadge

 

[KZO]

I'm glad it's back.

 

[Niewaznekto]

this author has a very... special type

 

[knthomsen]

Aw heck.. She's preprogonde now

 

[zazakmazak]

Shameless author, good for them.

 

[Idiot69]

how is babby form?????

 

[Dreamtiy]

Are these systoms of being pregarnt?

 

[Lqyi]

Innocent girl,i want fuck her

 

[Koneko_Nyaa]

Onee-loli... I'm in.

 

[bebopsushi]

Wait it just dawned on me now that I reread this is this an adult with a minor???

 

[Niewaznekto]

Wait it just dawned on me now that I reread this is this an adult with a minor???

There is other series with those characters, and in that series at least she looked to be at least late teen. Honestly, the series makes her feel as if she was mentally disabled.
EDIT: Checked, and she was apparently 17 when they met (not sure about the older one)