In the US (as with most countries) the IRS or whoever is the taxing authority requires you to report all income regardless of how it is derived.  And thus you are supposed to pay tax on cryptocurrency trades even prior to 2018.  Just because tax is not withheld doesn't mean its not owed.

Look at the recent news on the IRS and Coinbase (IRS tried to get all names and records and judge ruled they can get records for those with trades exceeding $20k for the year).  Might be a good idea to report your income now before they catch up as the penalties are quite high.

Just an FYI...