Rayleigh is also just as old, yet in his fight with Kizaru he didnt seem to think he lacked his strength, h only commented on how it had been a while since he used his sword. Yet he still fought on par with Kizaru. I dont think them being in their 70's gretaly effected their strength at all. Garp is still hurling cannonballs at people, and whitebeard was still the strongest person at Marineford (despite him being sick) aside from garp and sengoku.
Rayleigh clearly said that "
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" Indicating that he would have beat the living hell out of Kizaru and helped SH's if he was younger.If you think Prime Grap = Old Garp and Prime WB = Old WB then you need to improve your common sense.