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+)