It's probably os or in os collection. Read very long time ago that I can't remember the plot. It's maybe an OEL..i'm not sure. Some details (not sure about those too ;____; ) :

There's an old man and a little girl. Girl asked old man why he's old..she wanted someone at her age..bcs of the age gap, childish girl got mad at old man

It turned out that she's a robot created by old man (or given to him) when he was a child. They had good time together, promised to be friend forever. But an accident happened. It took a long time for him to repair her. he became an old man. She awaked but lost all her memory

Human age as time flies and..yeh. But that robot girl is always a child.

Thank you.

Omg finally found it myself in some minutes with the tag android/s.

It's Meria. Thank you MU and this community 😚

Edit: I added some more tags. It's not exactly what I described in the first post. It's a bitter sweet touching oneshot so pls don't mind some wrong details from my first post and enjoy this story ^^