I was planning on brewing tomorrow. I made a yeast starter from a smack pack this morning. My scheduled changed and I have time to brew tonight. Can I brew tonight and pitch the yeast starter 6 hours after starting it?
Its not ideal, as you will likely have grown all of the cells you need, but it is certainly much better than not doing a starter at all. I know some brewers who only pitch actively fermenting starters.
Some of it can depend upon beer style as well. What style are you brewing? I know for saisons, I have pitched an actively fermenting starter just to help drive some phenolics in the final beer.
Can you give some more info on the beer you are brewing tonight? (OG, size, style, etc. and what yeast) My vote though is likely to go for it and pitch. By the time you cool (which you could even do overnight if you sanitized properly), your starter will likely have totally fermented out. I’ve had some starters ferment in a matter of 5-6 hours. Are you on a stirplate? If not, just shake/agitate it as frequently as possible once you get home before/during brewing.
If your schedule allows it, you have another option: brew tonight and pitch in the morning. If you’re careful about sanitation (which of course you should be anyway), this should work just fine.