B/X D&D does not use the age categories that 1e AD&D used, but it does note that the listings in the Monsters section (pages B33-34) are for “average-sized” specimens and that dragons generally range from 3 hit dice smaller to 3 hit dice larger than average.
Over the past ten years, I’ve made a point to put the “dragons” back into “Dungeons & Dragons” and adventurers in Kilgore’s B/X game need to be prepared to run into the scaly beasts at any point.
Of particular note, a mated pair of black dragons is currently rampaging around the region where two different sets of PCs are adventuring, with both groups party to the situation. And one retainer proudly wields a sword and shield scarred by the acid of a close encounter with that pair’s (late) offspring and a saving throw barely made.
Here are the numbers I use for the full range of basic B/X dragons at each of the seven sizes, taking the stats on B33 and adjusting them up and down three HD. (One thing I am not really sure about is the magic spells…dragons in my game do not usually use magic even if they can speak.) I’ve adjusted damage, breath weapon size, and other factors up or down along with the hit dice.
I should also note that unique dragons also exist in my game. If I need a special dragon for a special reason with special characteristics, I just write it up. I don’t feel the need to make Smaug fit within the numbers of a basic B/X dragon…Smaug gets his own numbers and abilities. The same goes for special cases for all types of monsters, but it goes double for dragons.