Speech/language therapy

How old was your child when they started speech or language therapy? What were some signs that made you think they needed to be evaluated? My son will be 2 on April 29th and although he uses his own language most of the time I cannot understand him. He is very confident with some words and repeats them frequently everyday, all day, but he seems to be delayed in learning more words. He also puts a “b” sound at the beginning of many words such as “bred” for “red” and “bwee” for “wee”. He walks around babbling to himself all day so I just assumed ok any day now he’s going to start spitting out words, but the closer we get to his second birthday the more concerned I’m becoming. Any advice or tips would greatly be appreciated. I just want to get him help if he needs it or stop worrying if it’s normal development.