Age alone does not decide it. Well-maintained engines of considerable age cross oceans; neglected young ones fail in the first week. Gather evidence before spending.
Assess mechanical condition where it is warranted — compression or crankcase pressure testing, and oil analysis trends across several samples rather than one. Inspect the parts owners rarely look at: the exhaust elbow and mixing point, the heat exchanger, the raw-water circuit and everything aluminium below the waterline. Corrosion in the cooling and exhaust path strands more boats than worn bearings do.
Then assess supportability. Can you get filters, impellers, belts, gaskets and a starter for this engine in the places you intend to cruise? Parts availability, not condition, is what turns a minor failure into a months-long delay in a remote port.
Finally, assess the installation: mounts, alignment, wiring, fuel system, ventilation and the quality of previous repairs. A sound old engine in a poor installation is a worse bet than the engine alone suggests. Where the answers point to a repower, the surrounding systems usually need the budget as much as the engine does.