library(data.table)
dt <- data.table(sys_id = 1:9,
             A = c(11, 12, 12, 13, 14, 15, 15, 18, 18),
             B = c(2, 1, 2, 1, 2, 1, 2, 1, 2),
             Age = c(13, NA, 13, NA, 11, NA, 12, NA, 12))

sys_id  A B Age
1:      1 11 2  13
2:      2 12 1  NA
3:      3 12 2  13
4:      4 13 1  NA
5:      5 14 2  11
6:      6 15 1  NA
7:      7 15 2  12
8:      8 18 1  NA
9:      9 18 2  12

dt[, Age := unique(Age[!is.na(Age)]), by = A]
dt

sys_id  A B Age
1:      1 11 2  13
2:      2 12 1  13
3:      3 12 2  13
4:      4 13 1  NA
5:      5 14 2  11
6:      6 15 1  12
7:      7 15 2  12
8:      8 18 1  12
9:      9 18 2  12