I'm of the mindset that you can't be too old to enjoy RPing. You may not have the energy or time to write as often as someone younger, but you can still have the motivation to try. As far as a minimum age, I think as long as you're able to write coherently and you make it clear how old you are...
Not angry necessarily, but very sad. Just had an RP partner/friend message me to say she was too busy with RL stuff to RP again ... but then just said goodbye and blocked me. I'm still distraught about it.