I am 70 years old. I ride a Trek 7000 that I bought new in 2005. My wife rides a Trek 7000 step through frame bike I bought for her in April 2011.
My 7000 now has over 10,000 miles on it.
This is a 17.5 inch frame bike.
I stand 5'10" tall.
My inseam is 30 inches.
I have about 5 inches of seat post showing above the top of the seat post tube.
When I sit on the seat only the balls of my feet touch the ground. Seat height is adjusted to me me the correct angle on the knee at the bottom of the pedal stroke. Both knees have degenerative joint disease. Some tissue hardened and some frayed in the knee joints. Knee angle during pedal rotation is critical when it comes to my knees on a long ride. A short ride for the 2 of us is 30 to 40 miles a day. A good ride is 50 to 60 and last year we did 70 miles in one day. No problems after the ride other than being a little tired.
With my wife's Trek 7000 I just had the elcheapo stock Shimano rear derailler replaced. She put 2,500 miles on the bike in 14 months of use. Rode right through the warm winter. Replaced with a more expensive and durable model. Also replaced the original freewheel and chain at the same time.
You comment about possibly looking for a seat that will go a little lower.
You might want to look at dumping the stock seat post with the spring in it. Once they start to wear you will find the nose of the seat shifting from side to side as you pedal. They use a plastic washer with a key slot that wears. I put a regular no spring seat post in mine. That would give you a bit more adjustment in getting the seat lower if needed.
I finally had to replace the rear wheel on my 7000. Hub bearing races a bit worn. Spokes had reached the end of their useful life. Had two break while riding. Cost me $64 to replace it with a newer Trek rear wheel. Given the 10,000 plus miles I can't complain about that. A rack and big bags on the back. Some days I am using it for a mule in hauling groceries or beer.