dooley

Aluminium will fail eventually, how soon depends on how it's used. But don't make the mistake of assuming steel is indestructible, some people are pushing steel so far in an attempt to compete with aluminium on weight that failures aren't unheard of. I've seen newish steel road frames fail catastrophically, and my Pashley trials frame definitely wasn't indestructible.

I don't know much about Dean Jester bikes to comment about your particular frame but as long as there are no visible cracks the flexiness is almost certainly no worse than it was when the frame was new. You'll just have to trust that the designer took the fatigue limit of the material he used into consideration.