The Weasleys (update on JKR's page)

[Erin]

No: HPFGUIDX 115155


> Finwitch wrote:

So now we have 10-year-old Bill, 8-year-old Charlie, 5-year-old
Percy, 3-year-old twins, 1-year-old Ron and baby Ginny.

<snip> There's the baby, the schooling of the three eldest and the
mischievous twins to take away most of Molly's attention. Is this the
year when the infamous Fred turning Ron's teddybear into a spider
happened?

Erin:
It is not.  Ron tells Harry and Hermione he was three when Fred 
changed his teddy bear into a spider.  Making the twins five... still 
a remarkably young age for such a feat.

I wonder about that incident sometimes.  Ron makes it sound as though 
Fred did it deliberately in revenge, with malice aforethought.  I 
suppose it could have been wandless magic of the sort we've seen 
small children display, but it seems awfully advanced for wandless 
magic.  Bill would have been in second year at the time.  Perhaps it 
was during a holiday and Fred somehow got ahold of Bill's wand?

Erin