Wide forward is actually one of those positions where players can reach a high peak very young. Especially if they rely heavily on pace, which will only decline with age.
Look at Olise, Mbappe, Coman, Doku, Vinicius, Rodrygo, Yamal, Barcola, Bakayoko, Palmer, Saka, Dembele. All a very high level by 21.
It's not like striker where you need to wait til their late 20's for them to properly learn the role. DM arguably another one of those positions too.
Re Brennan, it's a judgement call and I know it divides opinions, I just don't see him improving from this level. He's not suddenly gonna turn into Olise and learn some composure and technique.