Oh, yeah. The key is the date you became a lieutenant (or a captain), not how old you were. The British Navy was all about seniority. If you had more years than all the other lieuts, you were First, automatically.

So a 25 year old who'd passed his exams at 18 (the youngest most of them could manage it, since you needed to have been at sea 6 years before you could even try) could have seniority over a 30 year old didn't pass his until he was 24.