What's the latest a plant can start showing sex?

Is it normal for a 5 week old plant to not show sex? My friend wants to know.

Yes , it is normal, a plant will show preflowers when it is mature enough to flower. If you cut the light cycle to 12/12 is will show within a few days to a week in most cases.

You can tell your "friend" that If the 5 weeks is 5 weeks into the vegetative stage of growth you would not expect a plant to show its sex that early but if you veg long enough plants will show their sex while in the vegetative growth stage.

If you switch to a flowering light cycle, as in if you were asking if 5 weeks into flowering should plants show their sex, it can take as long as two weeks for the hormone that is released by the light cycle change to fully kick in and if plants have not been vegged long it can take two or more weeks at times into flowering before plants will show their sex.

i jsut started a seed and probably after a week or less its showing sex, i didnt mean todo that though lol..

After only a week? I doubt that, no offense. I've never heard marijuana showing their sex as early as 1 week, but I know that most plants start to show pre-flowering signs as early as 4 weeks.