At 61 and a bit portly (overweight) will I stand out like a sore thumb? Which is the best club near Frankfurt Airport?

No problem, there will be younger guys but also plenty of similarly aged guys. A few times I've seen guys with walkers, that moved at the speed of tortoise. He must have been well over 80. I saw him as he was going to a room with a twenty something btw.

You best bet near the airport is Sharks in Darmstadt. When I go there, I often stay at one of the airport hotels, which leaves me less than 15 minutes from the club. At the moment it also has the best line up in the area.

Just to add...  There are frequent trains from Frankfurt Hauptbahnof to Darmstadt.  It's about a 25-minute ride depending upon if you take an express or a local train.  (You can also catch a train from the airport.)  There is also a bus that runs from the airport to the Maritim hotel which is right next to the train station.  It costs something like €7.00 each way.  Once you get to the train station, it's about a 30-minute walk or a 5-minute taxi ride.  I don't recall a tram going near Sharks, but I'm pretty certain you can catch a local bus from the train station if you don't want to take a cab.  

For the trains, you can use the www.bahn.de site.  There is an English option which makes things very simple.  And, yes... German trains run on time!

Ive been away for a while, but this is good info; thanks