Lindsay Smith didnt debut in NRL untill age 21. He played 1 game when rested everyone. At age 23 he played regular NRL. 2yrs later he played for Australia.
The indication for being an NRL player isn't necessarily playing NRL by 21, but playing regular NSW cup by 21 (there are always exceptions but it's pretty rare if a player doesn't).
Smith is poor example. Brown resumes is poor in comparison to Smiths
Smith was a regular top 13 starter for panthers flegg at only 19. Brown barely played SG ball at 19 for West's.
Smith was then starting prop for panthers NSW cup side in round 1, at only 20. Covid then killed the comp
At 21 Smith started in 11 NSW cup games before Covid killed the comp.
If Covid didn't kill the comps, Smith probably has played 40 nsw cup games by 22 (assuming no long term injury). And maybe a better chance of debuting earlier then he did.
That is type of resume you want of player to indicate they may have genuine potential to be an NRL player. Not only one NSW cup game by 21.
BTW, I hope Brown makes it, and he did good in the trials! And obviously he has impressed Ryles and co.