In biology, a half-life is the time taken for a substance to lose half its effects.
In software I think of a half-life as the time it takes a piece of knowledge to become half as useful.
The problem I see often in developers is that they don't consider the half-life of their knowledge. Many new developers mistakenly assume what they have learned will be useful many years later.
Mistake: developers don't strategically consider what to learn and they don't develop the skill of learning quickly.
The faster the pace of software changes, the more valuable the skill of learning quickly becomes.
Being able to learn quickly has let me write applications in the LAMP and MERN stacks, as well as on Android and IOS.
My tips for learning quickly:
Understand the commons patterns and architectures. These repeat across time and environments.
Be able to run experiments and tests. If you can test well, you can understand just about anything even if you don't know the syntax very well. And in the future, you will not know the syntax of some language or framework very well.
Spend more time reading other people's code and documentation. If you can read other's code well you can incorporate their ideas faster.