Should I wait until level 20 to decrypt exotic engrams or does it not matter?
it will decrypt at a higher light level than your current max, so... decrypt wheneer you want. the longer you wait, the higher the light level will be - but the longer it'll take you to get there since you don't immediately get higher LL gear. that's the tradeoff.
(given that you'relikely going to get more engrams along the way, i'd recommend just decrypting it whenever you get it, no point to already hoard things really, since you're REALLY far from cap)