iPhone X life expentancy

We can't say for sure since we are not Apple, but I would imagine that it will continue to be supported for quite some time.

The 6s is the oldest model currently supported with new iOS updates. The X undoubtedly has at least a few more years of life left in it. The 5s either is, or will shortly become unusable due to carriers shutting down their 3G networks and requiring support for Voice over LTE going forward.

It seems at least for now, the issue of iOS support isn't likely to be different for the iPhone X. The easiest difference to spot in the specs is that RAM went from 1 GB to 2 GB in the iPhone 6/6 Plus to iPhone 6s/6s Plus and that seems to be the dividing line where there was iOS 13/14/15 support vs being stuck on iOS 12. There are no guarantees, but it seems reasonable that there will be support for at least the next full revision.

However, there's really no telling when physical hardware will fail. I have yet to see an iPhone in my family fail due to hardware faults, but of course I've seen batteries degrade, which is to be expected with any rechargeable battery. Apple makes it pretty simple to replace, although there are caveats to the replacement process if there's any physical damage to the device that would need to be repaired or which would mean Apple wouldn't replace just the battery.

However, devices can fail randomly - especially as they get older. That just happens, especially if a device has been dropped over time. And there's no telling when an accident might lead to a malfunction. Even drops/shocks that don't seem catastrophic may lead to eventual malfunction. However, you plan to pass it down seems like a reasonable idea. However, there are no guarantees that it won't malfunction, even with a brand new device.

I agree with KiltedTim‘s excellent advice.

The iPhone X is a remarkable device that ushered in a new era of Face ID and a brand new design that is in use today with many more years ahead. 😉