If you start at 30 you can't expect to do numbers outside of a niche audience. All these late-30s rappers have had lots of fans and success for 10+ years.
Even Benny the Butcher has been releasing mixtapes since he was in his mid-20s.
That's called an outlier. Plus, they don't do record sales numbers and get by more for their marketing and merchandise. Griselda is barely doing 20k first week even with Roc Nation and Shady backing them.
If you're 30+, you best to have some good side-hustles besides rap (social media, merchandise, acting, etc). There is a definite ceiling as far as record sales, unless you're basically rapping like a youngin and hiding your age.
Yes, in music you can be any age to do it. But in the "music business," you have to be a good investment and the profit just isn't big with a rapper who is just starting or hasn't made any major connections at 30+. You're high-risk low reward