Let C- children, T - teenager, YA - young adult, MA - middle aged adult, S - senior citizen.
Question provides 1st 3 seats from left as - YA, T, YA
Question also states S will be in first or last seat. Given above, S will be in last seat.
So far arrangement for 9 seats is : YA, T, YA, _, _, _, _, _, S
Its also provided that MA >C, T, YA, S and T < C, YA, MA
Since no two individuals with same age group can sit next to each other 4th is not YA and 8th is not S.
Since MA > YA we need atleast 3 MA which also cannot sit next to each other.
So arrangement for 8 seats is : YA, T, YA, MA, _, MA, _, MA, S
Since T < C, and we already have 1 T, we need atleast 2 C's which are remaining seats.
Fifth individual: Children
Seventh individual: Children