I think it would be cool if there was a way to keep baby animals from maturing.
This would allow you to have a mix of babies and adults in any pet room/ animal sanctuary you might create.
One possibility is that when you put a name tag (or a craftable item based off a name tag) on a baby animal, it would prevent it from growing.
Hi, I did this on fabric 1.16.4
If you need a forge solution (uploaded it saturday)
Gamer, Minecraft Mod Developer, Creator of many mods that can be found in my CurseForge profile. (Helped port Grimoire of Gaia to 1.8.9 and helped getting a 1.12.2 version going)
1.15 forge has https://www.curseforge.com/minecraft/mc-mods/forever-young