The answers above are correct.
Be aware that some troops adopt policies that prohibit scouts under First Class from working on MBs. (There is usually an exception for summer camp.) I have seen it twice. I strongly oppose this, and it may even be contrary to national policy. I don't know. But be aware that some troops are really whacked about this and that you might see or hear some people parrot that in conversation because they don't know any better.
This is the very tricky part of scout-mastering.
Sometimes a troop/patrol needs to lean one direction or another on advancement because of a strongly felt need. E.g., you may really need scouts to master 1st class skills for them to meat a goals of hiking and camping in a particular region. Thus you push everyone towards that rank, because the more scouts without basic camping skills, the more you have to "dumb" down the program for safety's sake considering where the boys will be and what they'll be expected to do.
On the other hand, you might have boys who want volunteer at the fire house and swimming pool. Well, then you might want to lean on everyone getting Fire Safety, Swimming, First Aid, and Lifesaving merit badges, and let trail to 1st Class be a focus on only a few weekends and meeting nights per year, again for safety's sake considering where the boys will be and what they'll be expected to do.
Needless to say, the next guy in line might hear either strategy (e.g. no MB's until 1st Class, or no 1st Class until MBs) as a hard-and-fast rule handed down from on high, when a plain reading of the program materials encourage flexibility and allowing boys to advance at their own pace.
That's why I strongly encourage scouts to read their handbook for themselves. It really is the first step in teaching a skill successfully.