Demo accounts are VERY useful for new traders. Especially for strategies on the day & swing time frames. Shorter term, and the inaccuracies of the simulation may be too big. Longer term, and you won't get a big enough sample to mean anything.
If you can't consistently make money in a demo account, there's no hope of doing it in real life. The opposite isn't true though - you can make money in simulation and lose in real life pretty easily for technical or psychological reasons.
If you can't consistently make money in a demo account, there's no hope of doing it in real life. The opposite isn't true though - you can make money in simulation and lose in real life pretty easily for technical or psychological reasons.