I wonder if you can identify the age of devices based just on network scans? Like if an advertiser can tell I've got a washing machine model that was last sold 7 years ago, it's time to spam me with washing machine ads.

Quite probably. nmap does (or certainly used to) have options to report the OS of a given machine had based on various quirks of the packets it got back from them. It was disturbingly accurate at times. You put that together with banner messages from running services, MAC addresses, responses to broadcast packets, deliberate probing, and the number of devices you _can't_ remotely identify on a network without actually logging into them is probably very small.

It's a lot easier to create an embedding with equivalent information than to reliably identify a particular device, and that embedding is more than sufficient to enable targeting.