I often hear people saying 'the earlier the better' when it comes to learning languages. But it's a more complex issue that needs some unpacking, rather than a universally applicable statement of fact.
A really important thing to consider when thinking about 'the earlier the better' argument, is what you are measuring.
If it's how long it takes someone to learn a language, then it's actually quicker to start later when cognitive (thinking) skills are more highly developed and you can employ strategies such as reading, and prior knowledge and metalinguistic awareness, to help you learn a new language.
Adults learn more new language in an hour than babies, for example, who take years to make the same progress.
Children who start learning a second language when they start or change school will all finish secondary on a similar level to each other, whether they start in kindergarden or the end of primary school.
The only really measurable difference might be how 'native' they sound, not how fluently they speak or their vocabulary or understanding of grammar.
If you'd like to learn more about whether 'the earlier the better' is true, listen to episode 2 and episode 20 of The Language Revolution podcast, where I ask neuroscientist Dr Thomas Bak and multilingual education lecturer Eowyn Crisfield if 'the earlier the better' is really true.
There is interesting research in educational settings by Professor Florence Myles and the RiPL network too. You could argue that 'the earlier the better' is true for infant simultaneous bilinguals who are fully immersed in their languages.
But the same adage does not apply in early primary school where children have less well-developed cognitive skills than older learners, as they are still learning how to think in an age-appropriate way, and they also have a lot less exposure to the new language(s), unlike babies who are immersed.
On average, primary school pupils in England receive 30 minutes per week of French in a non-immersive setting, often without a fluent speaker.
This is why approaching 'the earlier the better' with a bit of caution is useful – it is not universally true!