Is it normal to become lactose intolerant at 20?

Yes: As we age, people tend to become more intolerate to lactose.

Can emerge anytime: LI can be something you are genetically programmed to have or acquire transiently. A bad case of diarrhea will often trigger it due to the intestinal injury, but that heals over time and the lost enzyme returns. When it emerges depends on dietary factors (lactose load) and age. Some high lactose products like milk are more likely to trigger symptoms than low lactose sharp cheddar cheese