Obviously I think the answer to this question depends so much on individual circumstances that all any of us can do is offer anecdotes. I think that while energy levels do decline as you get older, the degree of the decline depends largely on how much you stay in shape. My partner and I are very active and find ourselves only marginally less physically energetic in our 30s as our 20s. I've seen friends of ours with more sedentary lifestyles having a much sharper decline. If you're inclined to stay in shape then I don't think age makes as big of a difference (within reason.) But YMMV.