Hi @newuser162! As you are new to the community, when you get a chance, please introduce yourself here.
I’d also encourage you to add a real picture as your avatar because it helps build community!
For your question, this is definitely doable! Once you have entered the roster, you can then do a query to find only the members who are 15+ and use that as the values list for your select multiple.
We do something similar here, where we collect name, age, sex, etc. in a first form:
quest1_03_0.xlsx (56.7 KB)
and then in a second form we have queries to identify mother or father, etc.
quest1_03_0_update.xlsx (56.4 KB)
You can see in the queries tab how we use “selection” to show the list of opposite-sex individuals, for example, in roster_spouse. You can do something similar with your variable for age being >=15. Note you will need to make sure to also use the household identifier – for my example it is q100 – to make sure that you have only members of the same household.