If you develop a smash style, hard hitting style now... who's to say that you won't still be hitting a harder ball RELATIVE to the other 60-70 yr olds you play in future years?
You won't be hitting as hard as you do now, but you might be hitting harder then most of the oldsters you'll be playing against then... with the benefit of them being less mobile.
If anything, you're heavier hitting might be even MORE effective as your opponents age... especially on serves if you have location too.
I'd say develop the style that suits YOU now... your game will evolve as you do. AND as you develop your game now, won't competancy in all areas/styles suit you well today also, and down the road?