For really small kids, 20 degrees and up with 30 and up being ideal. No snow or wind preferable 

For older kids, 15 degrees and up with 25 and up being ideal. Snow fine but wind can be hard

For serious skiers, 0 degrees and up and many prefer 10-15 degrees. Snow and wind are both fine as long as visibility isn't too limited. 

That said, spring skiing and corn snow in the 40s is fun for everyone 

Take this FWIW from a former ski instructor and race coach