Single ply roofs often come with 15-20 year guarantees, but they can last 25-40...

https://claytoncllo171.tearosediner.net/why-do-roof-problems-show-up-after-two-winters

Single ply roofs often come with 15-20 year guarantees, but they can last 25-40 years in real life. The guarantee covers basic defects early on, not wear and tear over decades