Not an expert on this but my understanding is that at 2+ risk starts to increase, especially that have seen high use and fast charging.

My layman understand is that dendrites accumulate over time so risk is incremental over use...and for phones use and time is basically same thing. Low use items I'd totally run for many more years.

I'm currently using an iPhone 13 from late 2021, so almost 4 years old. It's showing 79% Maximum Capacity in Battery Health, and I only occasionally use more then 75% of battery over a days use.

I still have the iPhone XR I upgraded from, so a 7 year old 2018 phone, that still holds a whole day's charge too (but doesn't get much use, and doesn't have a SIM in it right now so I'd guess it's powered down the cellular radios?)

I have an iPod Touch from a bit before Covid, so 6 years old from 2019-ish - it stopped getting daily use when Covid and WFH hit, so it's battery is old, but still in reasonable condition. (Pity newer iOS won't run on it...)

No. We have a couple hundred phones at work that are 5-9 years old. Does failure rate increase with time? Of course. Until you hit 5 years, the phone is perfectly fine.

I think it's charge/discharge cycles more than time that gets to batteries. I've heard that a decent rule of thumb for LiPo batteries is that they'll drop to around 80% of "new capacity" in around 1000 cycles. So around 3 years to drop to 80% capacity if they're fully charged/discharged daily, or around 5 years if they're only discharged to 30-40% and topped up daily. If you only charge them once every week or month (like my Kindle), the battery will probably still be fine after 20 years. And on the other hand if you're the sort of person who talks on your ophoine for several hours a day (like my boss) or streams YouTube all day - and need to recharge your phone in the middle of the day, you'r battery could drop below 80% in a yeah and a half.

Depends on how you use it, but with intensive use like 2+ charge cycles a day 7 days a week, and long times connected to charger makes 2+ years quite old.