You are younger than I was when I started dancing (28 years old). Yet, after 5 years of training in one dance style (Jive), I was able to place 4th in national competitions in Switzerland with a partner that I trained myself from scratch (she was 16 when we started as partners, a double jointed gymnast with an unbelievable elegance and grace). Of course,I was coached by a 3 times world champion who charged his best students a ridiculously low fee. Obviously my partner and I trained a minimum of 9 hours/week. You see: I am living proof that age does not matter!

You're never too old to dance, especially if you're talking about social dancing. One of my current instructors didn't start until she was past 30, and she moved on to competitions....

So far as what to start with -- it sounds like you are already within a social circle where people dance. If so, and if these are the people you'd be dancing with -- ask them! Learning to waltz would be of minimal benefit if all they do is Argentine tango or a free-form "bump & grind."

If you don't know people who are already dancing, talk to your instructor as to what dances are popular locally with the group you are interested in. Good luck and have fun