Double NAT on Airport Express

I have 2 questions that have risen when addressing slow internet speed tests. First, I have a newer Comcast Gateway Modem Model: TG862G (about 5 months old). I'm using an Airport Express (5 years old - July, 2012) with Airport Utility Version 6.3.6 (636.5) with an iMac (24 inch, early 2009) 2.93GHz Intel Core 2 Duo, El Capitan 10.11.6 and also MBP, Sierra, 10.11.4 (13 inch w/Retina Display, Late 2013).

1. Do Airport Express routers deteriorate with age like many routers, possibly slowing down internet speeds? Mine is 5 years old and some say replace it.

2. I have been running just the AE for wifi placing the Gateway on Bridge Mode. I noticed internet speeds were down between 12 and 25 mbs. Comcast sent a notice that they doubled my internet speeds but when I followed instructions for rebooting the modem to get the upgrade, the speeds didn't change. When I called them, they said the upgrade could be implemented because I was in Bridge Mode. I turned Bridge Mode off on the Gateway and the internet speed tests on the Gateway network significantly increased. I tested the AE network and found they significantly increased as well, but I get the flashing amber light and the Double NAT warning advising me to put the AE in Bridge Mode. Both networks are totally functional. The Gateway router is in the basement with great speed but it decreases as I move upstairs and to my outdoor deck where I like to work. On the deck the speed drops to 7.2 mbs, barely useable. I like using the AE router, located upstairs in the kitchen and in close proximity to my deck, because I get a good signal. The internet speeds are acceptable, with the Gateway overall faster (except on the deck as noted). QUESTION: Can I keep both networks operational at the same time (checking to ignore the NAT warning on the AE) and be able to switch networks depending on location? or, should I take the Double NAT warning seriously and put the AE on Bridge Mode? Are there security concerns, connectivity or other problems that may result from keeping both routers operational?

Thanks! Rod