What makes it old? Is the CPU within the phone about to die of old age?

Seriously though, it's no secret that the problem with older phones is that the manufacturers refuse to continue supporting these with updates if they can't make money off of it. There IS the legitimate reason that these phones may not be able to use a particular LTE band that a cellular phone carrier has since come to use.

If there's no warranty to void then you may decide that you may have nothing to lose. That's why I updated my Galaxy Tab Pro. I was ready to buy a new tablet if I had bricked this one but I was successful and it never came to that.

It's a judgement call: Would you rather spend time or money to have something that's more up-to-date?