You can join one that might appeal to men over 50. There are many charities that welcome volunteers. You could also try the local golf courses to find men over 50.
Many marinas offer lessons and it can be an enjoyable skill for you to learn. If cocktail hour is not something you care to partake in, then try sailing lessons. Happy hour is a great place to meet men over 50 who like to sail the waterways. If you enjoy the ocean and boating, you can frequent the public yacht clubs in your area.
Many single men in this age bracket can be found climbing the foothills of mountains or trekking through a forest.
If you would like to meet a man over 50 who loves the outdoors then you might try joining a hiking club.