It would be a few weeks earlier than I'd expect, but I've stopped thinking that anything is impossible with Nature in recent years. I've seen the prey species breed a few weeks earlier the last few years, so I suppose some Sparrowhawks could adapt over time. And then there's the Weather...
So, yes it's unlikely, but would it really be that surprising to see one fledged a few weeks early?
In fact I'd be interested to know what's the earliest recorded juvenile Sparrowhawk, but it's hijacking this thread