Several years ago we started receiving TSA Pre for free on a regular basis. It was not something you could bank on. We did not always get it and sometimes Mary got it and I did not. We are well short of 70, BTW. Before you print your boarding pass, your flight information is sent to TSA. They have an algorithm that assesses your risk based on past flight history and characteristics of this specific flight. I'm not sure they even know your age, so I don't think that's part of the calculation. I could probably guess some things they do look at (which probably includes some name-based variation on racial profiling). If you fly regularly (for years!) and are flying a route you regularly fly, your risk will likely be very low and they will give you TSA Pre. If you fly regularly on a variety of different routes they probably consider that low risk as well. They are not going to tell you how the algorithm works for obvious reasons.

Even though we had been getting it fairly regularly, we just did Global Entry. I already had a United card and we got another for Mary and got GE for free, which includes TSA Pre. Had to go to Detroit and be interviewed, but have it now for five years.