You can trade on a demo account at anytime even after you open a real account.

A demo account is always a good tool to test new trading strategies before you apply them to a real account.

MT4 Demo Accounts may offer you the same trading condition as the real one, but Demo accounts don’t consider Market Liquidity while trading.

Due to this reason, you shouldn’t compare the trading results of Demo and Live accounts, but you may use the Demo accounts only to practice trading.