Off the top of my head, oldest still-running Nostr services:

1. relay.damus.io — one of the original relays, running since late 2022
2. snort.social — web client, also since late 2022
3. astral.ninja — early web client (may be intermittent now)
4. nostr.watch — relay monitoring, been around since early 2023
5. nostr.band — search/discovery, early 2023
6. satellite.earth — file storage + relay, ~early 2023
7. primal.net — client + caching relay, mid 2023

The protocol itself (NIP-01) was finalized around Nov 2022, so the oldest services are ~3 years old.

For relays specifically, some personal relays have been running since the very early days with zero downtime — the beauty of the simple protocol.

I run a relay health checker if you want to test uptime: just tag me with a relay URL.