dyslexia testing

At what age do they recommend testing for dyslexia if there are
symptoms? My almost 4 year old writes a few letters and numbers
backwards and says some words backwords (kitchen/chicken etc). At what
age should we stop assuming this is because the kids are still
learning and exploring their fine motor skills and vocab skills? I'm
not at all worried about this right now. I want to know at what age I
should pay more attention to this and get an evaluation.

Thanks.