Under ideal conditions, they will be bigger, healthier, and will flower and fruit earlier. Under the conditions most of us have, they will struggle to grow, become leggy, get stunted and not produce well at all.
Ideal conditions being a heated greenhouse (in my cooler climate) and having plenty of space and materials for potting up as needed.
[Why can you only start seeds 6-7 weeks before you want to plant them? I have my seed packets - most say plant indoors 6-7 weeks before last frost. What happens if I plant them 10 - 12 weeks?]