Does age influence the ability to learn languages? This question comes up a lot and is particularly relevant to golden-age language learners. There is not, as far as I am aware, an advantage or disadvantage conferred solely by age. Scientists have rejected this hypothesis. However, there are some conditions that typically go along with age and which impact language learning. I will try to list some.
Children are less afraid to make mistakes. This gives them an advantage over those adults who dread failure, but not over those who have retained the ability to enjoy experimenting.
Children have to learn a whole world while learning a language. This is a significant disadvantage, meaning that someone who studied a language for 4 years will generally outperform a 6-year-old native speaker in terms of verbal ability.
Children have less preconceptions, which helps with unrelated languages. Some adults have a really hard time accepting that e. g. Asian languages work very differently. Those who are able to suspend their idea of how things “should be” will learn faster.
Students know how to learn and memorize something. Adults may or may not have exercised that muscle in the last few years, so it may take them a little longer to get back into the swing of learning. This would suggest a peak of language-learning ability among university students, but significant ability among everyone in academia who has to keep learning. Unfortunately measurements wouldn’t tell you anything useful because language courses at university are also among the best-performing ones, catering to people who can learn and need to learn. Presumably IQ would also play into the results.
Adults already have a large vocabulary. Words like “democracy”, “Communism”, “Coca cola”, “sushi” and the like tend to be the same across languages, even many non-European languages use the same words. Children can’t use these cognates in their learning, but adults can. Depending on the language, you may find it easier to read an article on geography rather than a children’s book.
Adults may have learned other languages before. Even a short amount of time spent on another language (such as the 2 weeks learning Esperanto that “Irish polyglot” Benny advocates) can later help you make progress faster in another language. Same for an aborted attempt to learn a language at school. Children really have to start from scratch.
In my opinion, all these factors pale before time & focus though. Children have more time and adults have more focus. Language-learning is for the most part a function of the time you put in. While children tend to have more time, they also have less drive to spend that time learning languages. Rather than worry about how your age might theoretically affect your ability, start learning today.