Is there really no participant birth year data?

Since data freeze 11 (DF11) there is APPROX_BIRTH_DATE variable in the minimum extended phenotype data file.

This variable does not exist in earlier data freezes, but you can calculate birth year easily from all individuals using longitudinal data. Each event has the APROX_EVENT_DAY and EVENT_AGE, so by computing APROX_EVENT_DAY - EVENT_AGE you should be able to get birth year from any of your desired endpoints.