"Bluesky is younger than Nostr" may be true in terms of public launch or early development, but it is not entirely accurate without considering the contexts of creation and conceptualization:
The Bluesky project was announced by Twitter in 2019, with the goal of creating a decentralized protocol for social media. Although it has gained traction and started attracting users more recently, its roots go back to that 2019 announcement. That makes it a project with a few years of work behind it.
On the other hand, Nostr protocol was officially proposed in 2020. Nostr is therefore chronologically later than Bluesky's announcement, although it has grown rapidly due to its simplicity and compatibility with Lightning Network.

Thus, while the practical adoption of Bluesky may be more recent than Nostr, the original idea of Bluesky predates it. The perception of "youthfulness" depends on when one looks at both projects.