I use the same rating system as used for movies shown in US cinemas. However, the stories are rated at the time of their release (which may date back more than a decade) and therefore may seem harmless compared to today's standards.
In addition, I'm German. German standards are more sensitive to violence but more lenient regarding sexuality than the US equivalents. When in doubt, I always used the stronger rating.
The rating system in detail is:
General Audiences. All ages admitted. [G]
Parental guidance for children under 8 suggested. [PG-8]
Parental guidance for children under 13 suggested. [PG13]
Restricted. Under 17 requires accompanying parent. [R]
No one under 17 admitted. [NC-17]