Keeping formulas constant for only part of rows

Hi

I have longitudinal data in an excel file that includes participant ID's in multiple rows. For each row, the participant ID has an associated age which changes as the participant ages. For example, data for participant 4 (4443 rows) is recorded with the participants age increasing from 44 to 59.

I wanted to create a new variable of the participants age when they first entered the database i.e. for all 4443 rows I want their age to be derived from participants 4's first row (age 44). However, once it goes to the next participant (participant 7) I need it to record their initial age (48 in cell 4447) in cells 4447 to 9011. And so on with the next participants.

I have attached a subset of the dataset as it is very large. Please can someone advise on a formula that may work for this problem?

Thanks in advance.

Phil

Thank you very much Ali. This worked perfectly