Not likely in Michigan. Although Brussels sprouts are in fact hardy, that means they will survive cold temperatures, not that they will thrive in them. You don't list your zone, which makes accuracy in providing an answer more difficult, but what your plants will be dealing with if planted in September are continually declining growing conditions, and it is unlikely that you would see a substantial return on that crop. You would likely be best served by a mid-July planting; this would give the seedlings about 60 days of good sunlight and soil conditions to make a strong start. In mid September, light intensity and duration begin to decline, and the plants will slow down. According to my chart, first frost dates in Michigan can occur as early as August and as late as October depending on where you are, and although Brussel sprouts and many other crops can in fact withstand frost, it is not doing them any good. Finally, it is far more pleasant to be out collecting a meal from the garden on a sunny November day than digging through snow to find what the hungry mammals may have left for you in January.

I planted my transplants in the beginning of July but this year i would like to get them out maybe the beginning of june. The sprouts produced well when i planted in july but i would like to see if i can get the sprouts larger this year. I was picking them in October and November. I will be starting my seeds the end of April or the beginning of May this year. They are heavy feeders and liked the soft rich soil. I hope this helps.
Stacy

In Michigan, plant them no later than June 1st as transplants, or May 1st from seed. They take a long time to grow large enough to amount to anything. You might, might squeeze out a decent crop from transplants set out by July 1st IF the growing conditions are excellent.

And, as a rule, they are NOT hardy enough to survive a Michigan winter, except with significant amount of protection, and that leads to other issues such as rot, voles, etc.

Finally, if you do overwinter some plants, no matter what age, keep in mind the fact that they will then be vernalized, and will bolt to flower and seed in the spring, rather than growing vegetative growth.

I planted out seedlings in July two years ago (seeds started early June) -- no Brussels Sprouts.

I planted out seedlings in late April last year (seeds started in March) and got a great crop which we ate in December and early January after many hard freezes. That's for zone 5 and a very mild winter.

As everyone above has said, give them longer to develop. And guage it all according to your zone.