割付 is defined as allot, assign, distribute, divide among (Source: Nelson's Japanese Character Dictionary)

I feel that the word "allotment" is used more for inanimate objects. (As in: The students were alloted school supplies.)

"Distribution" is defined as: The act of distributing or spreading or apportioning (which fits as a heading in your context).

Therefore, I think that "Distribution" as you put in your question works well.

The below alternative is another possibility using the word "divided in the latter part of the sentence.
Distribution: Participants were equally divided by sex and age (12 groups x sample of 100)

Also, I'd like to add that 条件 may not be necessary, since it looks cleaner as a translation and is also "understood" by what follows the colon.