The MPAA system:

G – suitable for all ages
PG – parental guidance
PG-13 – for readers of 13 and older
(PG-15) – for readers of 15 and older
R – for adult readers (18+)
NC-17 – for adult readers (18+)

FictionRatings:

K – suitable for most ages – 5+
K+ – some content may not be suitable for young children, 9+
T – contains content not suitable for children (13+)
M – contains content suitable for mature teens and older (16+)
MA – contains explicit content suitable for adults only (18+)

My system:

Green – suitable for all ages
Yellow – suitable for teens and older (13+)
Red – suitable for adults (18+ or 21+)