40. Anyone under 40 is not an older single, IMHO. There is no right age to get married, especially for girls who have no chiyuv to get married (although it’s a Mitzvah if they do), and therefore can get married when they feel like it.
On the other hand, boys have a chiyuv to get married as soon as they are mature enough, unless they are learning AND marriage will interfere with their learning. I think that even in that case, they are only allowed to push it off until 24, and nowadays, marriage is not necessarily an impediment to learning anyhow (although in some cases, it may be).
Conclusion: For boys it’s 25 and for girls it’s 40.