I've eaten sorrel through most of the seasons, before after and during flowering (sparingly though, might I add). Loses it's taste a little toward the end of the year but other than that it's fine until the leaves redden at the autumn.
It's got a lot of oxalic acid in it though, just like rocket, so it's best not to overdo it. If you cook it apparently the oxalic acid is reduced, but so is the sharp taste.