How early can you get gingivitis? 40's? 50's?

Much earlier: Gingivitis can occur whenever teeth are present-- even in early childhood.

IF you have teeth: Young children to elderly can get gingivitis any time oral hygiene is inadequate. It is simply inflamation in the gums. If you mean periodontitis then it usually comes later but it is preventable with good oral hygiene and regular visits to your dentist, see your dentist.