I think that you can choose when to become a trader. The best time for you to become a trader could be now, when the price of the commodity you are trying to trade is cheap, or maybe when you feel like it.
While there is no timing as to the appropriate time to become a trader, it is paramount that a would be trader puts the right foot forward. That can be done by first taking quality time out, to understanding how the markets work and their special attributes. For instance margin trading is a step higher than the normal buying low and holding till price appreciates, which is what most cryptocurrency traders are familiar with.
Beyond that, to say the right time to become a trader is when the price of the commodity is cheap, is actually besides the mark and vague! I suppose you meant to say, the right time to actually execute a trade is when the asset is cheap. The question begging for answer then would be, what price is cheap and it would take a knowledgeable and experienced trader to be able to resolve this puzzle!