dass "pushing" nicht energiegeladen oder kraftvoll zu verstehen ist -- yes, that's my opinion. To me it's hardly even a verb expressing something that somebody does -- after all, you're not pushing against that age, pushing it away from you or pushing it forward ahead of you -- but more a description of where they are.
As pumpkin says (#29), "pushing 60/80, etc." is often used in a context of admiration for how someone still looks or behaves. This makes it easier to see "stramm auf ... zugehen" as a possible equivalent.
But this use of "pushing" also occurs in contexts where the person's age has unfortunate consequences. E.g.
It's not so easy to get a job when you're pushing 60.
She complains that she seems to be constantly at the doctor's these days, but she is pushing 80, after all.
I assumed that "stramm auf die ... zugehen" would not fit in such cases. However, it seems that there are comparable examples, such as the one I quoted in #24, where the people referred to are seen as too old to get promotion.
So my view is:
1. I don't think "pushing" is energiegeladen, etc.
2. However, it's not quite as colourless as nearly / almost / approaching.
3. It seems that "stramm ... zugehen" , while in many cases carrying an association of energy and resoluteness, is also sometimes used in a watered-down way, referring merely to people finding themselves, unfortunately, approaching a certain age.
So it seemed to me that English and German speakers might well use these expressions in very similar circumstances, and to that extent each could be a suitable translation for the other. But of course you would have to consider that suitability, or otherwise, in any individual sentence.
If, in English, you wanted something as "kraftvoll" as the German, you would probably have to invent your own formulation.
[Note to self (as Bridget Jones would have said): don't engage in discussions involving a German phrase you don't really know.]