I'm 52 and my girlfriend is 50. We have been dating for 4 months and living together for 3 months. How long should I wait before getting engaged?

Well, you both seem experienced by your age. Though you have only been dating for sometime, you should know if its the real thing or not. Also, its not like you guys are starting your lifes off like a younger couple. You should have your life in order getting ready for retirement. If you feel comfortable getting married then go for it, its a big commitment. You should both be able to discuss it before, bring it up see how she reacts.