[Potiron]

 Question.
    If I were to sow some onion seeds late and the onions only grew to pickle size, could I use them as sets next year?

[DD.]

No.

That would be there second season and they would just go to seed.

[A Reyt Tayty]

No.

That would be there second season and they would just go to seed.

How do the purveyors of onion sets grow their onion sets then DD?

[Potiron]

 Good question Reyt Tatty,how do they.

[potatogrower]

you might get away with it if you sow seeds now in tray and then slowly move them outside when they're about 5 inchs tall.

i'm just experimenting growing from seeds myself so still new to it all. Best started in September indoors with seeds

[DD.]

In simple terms an onion set is an onion that is started off from seed and then stopped before it reaches anywhere near maturity, so it has not had a full season.

The OP is talking in terms of letting it grow as far as it can and the possibility of it carrying on growing the next year to make a full sized onion. To that onion it's already had a full season. A set hasn't.