NotMeNotYou said:
Any age. Unless they have been released into the public domain, of course.

I'd be wary of using the term 'public domain' as just because an artist is uploading to FA or Pixiv doesn't mean that they are Public Domain. They are publicly avaliable, but the artists still own any copyright provided by being its artist. Basically if you don't own it and you cant use it to make money then it's not public domain.