Final answer:
Hydrogen is fused into helium during the Main Sequence stage of a star's life cycle, which is considered the star's 'adolescence' where it gains stability and energy from core nuclear reactions.
Step-by-step explanation:
The stage in a star's life cycle when hydrogen is being fused into helium is known as the Main Sequence stage. During this phase, a star stabilizes and derives its energy predominantly from the nuclear fusion occurring in its core. This is a crucial process that maintains the star's equilibrium over an extended period, which is why all stars spend most of their lifetimes in the main sequence. This stage can be colloquially referred to as the star's 'adolescence' or 'adulthood' in the broader context of a star's life cycle. As they age, main sequence stars eventually evolve to become red giants or supergiants, but only after the hydrogen in the core has been substantially fused into helium.