How to tell if my young starter is mature?
A good rule of thumb as to when a starter has enough oomph to raise a loaf is whether it will float in water.

I can take the properties of my starter pretty much for granted these days, but when I was making it from scratch I made sure to test a spoon of the portion that I was discarding for its flotation properties.

If you feel your starter is not making progress I would suggest two things:


Keep it slightly warmer. The method at https://www.sourdoughhome.com/starting-a-starter-my-way/ which I more or less followed, suggests a temperature range of 18-30 degrees C, so your 20 degrees is at the lower end of that.
Discard a lower proportion. If your starter isn't racing away and exhausting its food supply, it may help to build its population if you follow the SourdoughHome recommendation to discard only half of your starter at each feed and replenish with 50g each of flour and water.
Check that your flour is as fresh as possible and consider switching to fully wholemeal until you get more activity going on, the meal being more likely to carry the necessary yeasts and bacteria, apparently.
experiment with a different water supply, perhaps bottled spring water; some people report that chlorinated supplies are problematic for starters while others have no problem, there may be variations in the exact chemicals used.