Try
;with cte as (select *, cast(left(MatriculeSecuriteSociale,8) as date) as [BirthDay] from Salarie)
select *,
format(BirthDay, 'yyyy.MM.dd') as [FormattedBirthday],
datediff(year, BirthDay, CURRENT_TIMESTAMP) as Age from cte where BirthDay > dateadd(year,-45, CURRENT_TIMESTAMP) -- find people who is older than 45 years old