This system will take your basic D&D characters all the way to level 14. There is another version which will take your characters all the way to 36th level (see BECMI D&D or Rules Cyclopedia), however I have to say this version has its charms. Why, you ask? Because level 14 is high enough! It will take roughly 1.5-2 years of regular gaming for a party to hit max level, and let's be real, how many campaigns last much longer than that? Considering how even more modern D&D (like 5th edition) only goes up to 20th level, it's not that big of a deal. If you're looking for an OSR itch to scratch, definitely consider B/X!
[5 of 5 Stars!]