No. The belief that it is comes from comparing an adult’s first three months with a child’s first six years, which is not a comparison at all. Adults are worse at one specific thing and better at almost everything else.
What actually changes with age
Accent is the one area with a genuine age effect. After adolescence most people keep a trace of their first language in their pronunciation, and no amount of effort removes it completely. Vocabulary, grammar, reading and the ability to hold a conversation show no such decline. In controlled studies adults usually learn these faster than children, not slower.
Adults have advantages children do not
You can be told a rule instead of inferring it. You can look something up. You already understand how sentences work, what a tense is and what you are trying to say — a child is learning to think and to speak at the same time. You also choose your material, your schedule and your reason for doing it. None of that is available to a seven-year-old.
What children genuinely have is exposure and no dignity
A child gets thousands of hours of input and is required to use the language all day. They also make mistakes constantly without embarrassment, which is the real advantage adults lose. You can recreate the first with volume, and the second by deciding you do not mind sounding foolish for a few months. That decision is worth more than being twenty.
The number that matters is hours, not age
Someone starting at fifty with four hours a week will overtake someone who started at twenty and stopped. Age changes almost nothing about the arithmetic. The reason adults give up is rarely capacity. It is a busy life and an expectation of faster results than anyone gets.
It is not too late. It was never too late. The only thing you have genuinely lost is the chance at a flawless accent, which nobody needs.