Bitcoin is definitely a legal currency,but its legality ultimately depends on your country of origin/residence...
Few countries have actually cracked down on the system and stopped all its activities by their citizens and within their jurisdiction
So try finding out if your country is one of them,if not then you're free to start trading on the bitcoin and other crptocurrencies
Good luck
in countries that do not recognize bitcoin usually only prohibits bitcoin from being used to transact or to buy goods, but we can still do treding and we can still buy bitcoin even though as a future asset, we can still buy and sell bitcoin because of its bitcoin global and online so we just have to buy in a country that does legalize bitcoin, because we no longer need to use the services of a conventional bank or a state bank.