We have been to Bulgaria many times in the pasty two years, now we have a house near Varna. Being at 73 a bit older than you I am not quite so much into the night life and clubbing scene, however, our young Bulgarian friends tell us that if you do feel concerned their advice is to make friends with some of the local people where you are staying and they will be happy to join you on a night out and do their best to ensure you enjoy all the good things in Bulgaria.
In summer time I would suggest the Black Sea coast is preferable to Sofia and that end of the country.The night life in all of the tourist beach areas is quite lively.
Golden Sands, Sunny Beach, St Konstantin and other resorts north of Varna are very popular.South of Varna are places like Burgas, Sozopol, Pomorie and Nesebar that all have different attractions.
In addition to the many web sites you can access we have found one
http://www.individualtraveltobulgaria.com/ and spoken to Patrick the guy who runs it with a friend, you may find Patrick a useful point of contact unless Peter or Sam decide to delete the address in their editorial role as being advertising a business, in which case I think I could pass it on to you in a direct e-mail.
I do hope that we will get you back on this site again after your trip with your points of view to help others.
Enjoy your visit.