Agreed - if there's nothing inadequate about the old unit, just keep using it. They're not perishable goods with an inherent expiry date*.
I don't know the rest of your system's components, but if you're using it for typical home uses (gaming, browsing, office work) then it is likely that your old PSU has enough capacity.
Was there anything in particular that was bothering you about the existing PSU?
[Edit] There is perhaps a minor case for buying a new PSU even if the old one is OK: if you rely on your PC for critical work and cannot tolerate the downtime if the PSU develops a fault, and you have no spare PSU, then you might justify buying a new one to keep the old one as a spare; however, in this situation you'd also consider having an entire spare machine for redundancy, not just the PSU.
*Certain types of capacitors do age over time but should be good for well over ten years unless you're constantly running the PSU well over 40 degC. Power surges will degrade the suppression varistors over time, but this depends on number/severity of surges rather than a strict age.