I've recently changed from 10 power to 8 (but better glass).
8 power with a larger objective gives you a good compromise between magnification and light gathering.
I'm 50, but have good eyesight. Can't tell a huge difference in magnification, but the light gathering on my 8x42 Leicas is significantly better than the 10x42 Nikon Monarchs I had before.
https://www.youtube.com/watch?v=0oENTX3p7F0
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