I’m not aware of a site specifically dedicated to older athletes, but there’s plenty of info out there for older folks that could help you design a training program. Here are a couple of sites where I’ve found helpful info:

https://www.roadbikerider.com/

https://www.peakendurancesport.com/

Fellow oldster carbonfiberboy has posted some details about his training program somewhere in this thread.

i hope this helps - others will probably have better suggestions.