A practical way to decide this is to look at both risk level and information gaps rather than age alone. If the transformer is critical for your process, operates near its thermal limits, or has a history of elevated moisture, rising hydrogen, or unstable DGA trends, online monitoring becomes justified. Another strong indicator is when you rely only on infrequent laboratory tests and lack trend data between samplings. TOR-5 is most useful when early detection of slow-developing defects can prevent unplanned outages or costly emergency drying.