Oh, you definitely turn 25 after 24. Maybe when someone asks you how old you are, your response is "I'm turning <x>" - as I said before, I might be old fashioned. Ollie Watkins is 24. I would describe him as being 24.
Your view on what a good season is may be interesting. To quote the figures given by @villaglint (I can't be arsed checking them myself ):
19/20 - 50 games - 26 goals, 3 assists - 47 games at CF
18/19 - 45 games - 12 goals, 9 assists - 21 games LW, 20 RW, 3 CF
17/18 - 48 games, 11 goals, 5 assists - 36 games LW, 5 CF. 6 RW
16/17 - League 2 - 52 games, 16 goals, 13 assists - 30 games CF, rest all over the place
To me, that's 4 good seasons in a row, showing improvement each time. It's pretty much what you'd expect from a young player getting better at his trade. he's basically been involved in a goal every 2 games for his last 2 seasons, and a goal every 3 games at worst, for 4 seasons in a row. That's decent!
For comparison, Grealish's figures are:
19/20 (Prem) - 36 games - 8 goals, 6 assists
18/19 - 34 games - 6 goals, 7 assists
17/18 - 30 games - 3 goals, 6 assists
16/17 - 31 games - 5 goals, 2 assists
Similar sort of thing, to me. Improving stats over time, as he gets better at what he's doing. Really not very impressive figures when noted down like that - maybe you'd consider him a "no season wonder" - but around 1 goal involvement every 3 games, 1 every 4 at worst.
I just don't see him as a one season wonder at all. I'm amazed anyone looks at this at says "ah, he's only done it once". Sure, the most recent season is the one that is most relevant to any upcoming transfer and would naturally inflate his fee - and it should do, he's had a fantastic season - but it really is just a story of progression, at least statistically.