Well, from a male standpoint...it all depends on what your intentions for the relationship are. If you both are mature and know you're just having fun, you might as well skip all the silly courting rituals and waiting and get right to the 'mutual masturbation', as it were. Hence, the one night stand.
However, I've got to assume due to the nature of your question that we're talking about a REAL relationship, one in which both partners have a chemistry and see a potential future together...in which case the answer is, whenever it happens naturally...and by naturally (I hate to say this because I'm sure the fellas will scoff), I mean - you're both SOBER, aware of your actions, aware of the consequences, aware that no matter what you think, the act will forever alter your connection to that person (you're no longer 'just friends'), and aware of the responsibilities that will be expected of you as a willing partner in the relationship. Good, meaningful, memorable sex ALWAYS is based on both parties being mature enough to handle it. Age isn't as large a factor as emotional maturity is....so ask yourself this before you start stripping down...how many previous relationships has he/she been in? How many sexual experiences has he/she had before this? The lower the number, the more likely that your partner will consider the act of sex with you to be EXTREMELY meaningful and powerful, so just don't mess with their emotions, be a responsible adult...and seriously, try not to break any hearts. If you aren't sure you TRULY care about him/her and you convince yourself that if you just have sex ONCE and it'll solidify your feelings....you are NOT mature enough to handle it. Consider sex an extension of your emotions....is it love or lust...and if it's lust, make sure it's a MUTUAL lust...or else you may be breaking a heart that doesn't deserve it, just to get your jolly's. Might sound fun, but in the end, it'll really tear at your conscience....bad karma comes around.
Cheers!
~Pun