I usually start most of my seed off in March in a heated propagator for toms/peppers and cucumbers.
If its the English marigold then they are just left in the unheated greenhouse along with the beans, Stocks.
When I used to grow beetroot I'd plant them straight into the ground so they were not disturbed when potting on. same with carrots.

I don't go for the windowsills as they do tend to get leggy and drawn to the light and if the room is heated that adds to the problem. If you need to start them on the windowsill then later is better than sooner to reduce legginess.

Usually the packet will say when it is best to start them off and I usually go for the latest date they suggest.

Thanks ,Seaburngirl mistily same as I usually do .Think others starting theirs Now are ‘Newys’ doing home grown
Through Covid ,lots more gardening ,buying greenhouses etc And enjoying their new hobby I always Start my Beetroot in Greenhouse THEN transplant when about 4” high. ( Monty Dons tip ) For about 5 yrs doing & great success

Do you sow them in trays or in seprate little pots?

Agree with Seaburngirl. Last year I started my tomatoe seeds too early and they reached the ceiling. The smaller plants adjust quicker when you finally plant them out anyway.

I normally leave it until March, indoors & out.

As Pammie says I think the early birds are mostly Newbies who can't wait, followed in a week or so asking about what to do about their leggy seedlings ;-)

I try and advise if I see a question about when to sow, but normally get swamped by others saying they've sown theirs already so now tend to let them get on with it & hopefully learn from experience