My sweet peas have been out for a few weeks and have doubled in size already.
I sow them in pots in the greenhouse early January and normally put them outside when they are about four to five inches high, just the time when they are trying to find something to cling onto with new tendrils.
We rarely get frosts here and if we do they are very light ones.
But, I always thought sweet peas were a hardy annual and did not suffer from frosts.