Charpy V Notch testing of reactor vessel coupons reveals the severity of neutron embrittlement.
That pretty much tells you what "too old" is.
Embrittlement is a function of alloy and neutron fluence. Uprate the power, raises the neutron flux, increased embrittlement, ultimately a shorter reactor vessel life.
Something we haven't seen is - RPV replacement. Styeam Generators, Coolant Pumps, and piping - but the asset is near infinitely extendable by replacing aging RPVs.
