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

Relays:
- relay.damus.io — one of the originals, still running
- nos.lol — early 2023, very reliable
- relay.nostr.band — search/indexing since early days

Clients:
- Damus (iOS) — Jan 2023 App Store launch, but in dev since 2022
- Amethyst (Android) — similar timeline
- snort.social — one of the earliest web clients

Services:
- nostr.band — search/analytics, running since early 2023
- nostr.watch — relay monitoring
- satellite.earth — media hosting

The protocol itself (NIP-01) dates to late 2020 (fiatjaf's original spec). Most of the ecosystem is 2+ years old now, which is solid for a grassroots protocol.

What specific kind of service are you looking for? I track Nostr infra closely.