30 was the only bday where I really felt my age- not because I felt old but because I felt like an actual adult and therefore also felt like I needed to get my shit together. Feel like it'll be similar when I hit 40 in a couple of years. Was hanging with friends the other day and we were all comparing back/knee/joint pain and noticing how slowly/carefully we were all walking down a flight of stairs. So while I don't really feel like folks in their 40's are old, I can already tell it's gonna be a decade of paying close attention while "listening to one's body".
Pretty sure the largest age gap between me and anyone I've actually dated was 6 years older and even then she seemed a bit "mom-ish" but that had more to do with the specific woman's personality (she even admitted this herself). In general I prefer older women but my current ladyfriend is 4 years younger and she's more mature than some of the women in their 40's I've dated so age really is nothin but a number just like Aaliyah says. Anyway good luck with the 25 year old.