I think it also matters what happened during the intervening 15 years. If you built an app in 2004 and haven't touched it really since, then it should hopefully still be reasonably easy to follow along, even if the conventions might seem outdated.
15 years of active feature development on the other hand will likely be a major slog for any developer, regardless of experience. There's a lot that can happen in 15 years even if there were only 1-3 developers if they everyone takes the easy path since they know where all the gotchas are. It's only when new devs are added that the curtain comes down and the lazy abstractions (or lack thereof) are exposed.
15 years of active feature development on the other hand will likely be a major slog for any developer, regardless of experience. There's a lot that can happen in 15 years even if there were only 1-3 developers if they everyone takes the easy path since they know where all the gotchas are. It's only when new devs are added that the curtain comes down and the lazy abstractions (or lack thereof) are exposed.