Well the difference with age play, as I have heard (never done it/never wanted to), is that there is a big shift similar to subspace but you psychologically shift into a "younger" self. I have heard friends of mine say they felt a switch go off and felt much younger.

Age play isn't my thing, I have had spankings as a submissive adult from a dominant adult who would impose rules (nothing too serious) so I suppose domestic discipline that was non-sexual. I felt younger in the sense that I was being punished for behaviors by someone I saw as an authority figure with in a scene, which did help me submit and relax. But Age play seems a bit to intense for my liking.

As for motivation, I think it's another step of D/s for a lot of people. I guess going into a "young mind" space can cause someone to be more submissive, to get more subspace, etc. I think a Dominant person gains more control as well.