At what age is speech delay typically detected?

Mine altho can say few words, he mostly only babble! Only if he see certain things then he’ll say it else really all babble. He also cannot say mama papa yet. He’s 14m. Doc did say 15m if cannot say mama papa therell refer him but i trust my son. He’s still progressing. Even his ifc teacher shows no concern on his speech. He also imitate us talk. And he really monkey see monkey do.