16 years is very old for a mattress. Around 10 years, or whenever you start noticing problems and it's time to replace.
Mattresses are one of the higher use, more important items in your home. You likely spend around 1/3 of every day on it, and the other 2/3 of the day are negatively impacted if you don't get good sleep. So I don't see the point in cheaping on out a mattress.
The main thing I don't like about cheapo mattresses is the environmental impact, with VOCs a big second. Buying a cheap mattress has a huge petrochemical footprint and they take thousands of years to biodegrade.
So I just can't do it, will spend more for one made with sustainable materials: cotton, wool, wood, natural latex. A nice benefit of these materials is that they are way more breathable. Paired with a breathable cotton-wool comforter, and I sleep way better and don't wake up sweaty/clammy. The cost is higher (e.g. $2500 for the brand we currently have), but with 10 years of use our cost is around $21/month.
Our current mattress is 4 years old and is basically still like new. I think it helps that we don't use a box spring, which can cause sagging.