Matching ids across rounds in panel data

Dear Statalist,

I am working with panel data to look at the effect of timing of menarche on investrment into girls. There is variation in the age at which girls reach menarche. Early ages at which girls reach menarche is captured in round 2 of the survey (this is my early menarche cohort). All who don't reach menarche by round 2 comprise my "late menarche" cohort, i.e. they reach menarche after round 2.

However, all variables are not captured across all rounds. For example, I have children's height, weight and the household sibling composition from round 1. While creating the summary statistics, I want to execute a command that says "for the girls that reach menarche in round 2 (as identified by a child id) give me the average weight when they were in round 1". Basically, I need to match the childrens id's across rounds. Just adding an indicator such as "if menarche==1" does not work since it only captures data from round 2 which is when the menarche variable was recorded. I'd appreciate any help in figuring out the code for this.

Thanks,
Karishma