Some of the unspent outputs represent coins held in cold storage by exchanges, and other crypto businesses ("Websites").

When a Website receives a withdrawal request, they will first check if their hot wallet can process the request, and if so, will process the withdrawal via the hot wallet. This is regardless of if the customer has had coins on the Website for a day, a month or a year, or longer. Being that there are risks involved anytime cold storage is accessed, many Websites will have ample coins in their hot wallets so that it is not necessary to access their cold storage very frequently.

The above means that the UTXO that are "old" will be artificially high.

The LN protocol (and similar protocols) will also cause many UTXOs to "age" because LN channels do not ever need to close, and may stay open for years at a time, even if coins move between the parties many times.