

Neither do the other thousands or millions of people who do so, but it’s not a particularly uncommon thing and it’s likely enough that (birthdate) is the reason it’s there. I’ve got my birth year (not '88) in my username for some sites because the bare username was already taken
It really depends on the mods. Stock Minecraft and even many mods would run fine on a potato, but some of the bigger mods actually require quite a bit of CPU/overhead.
There are also various mods that require certain (older) versions of Java etc to run, and those versions of Java may have known security issues.