Why does autism sometimes only visibly appear later in life? From what I know about the autism spectrum, a lot of people who were diagnosed later in life (not during childhood) didn't "seem autistic" until adolescence or young adulthood. Of courses, they had symptoms all this time but it didn't become obvious until later.

Why is that? Do you have some articles/youtube videos that I could read/watch to learn more about this (English or French)?
It is important to remember when reading this question and answer is that Autism Spectrum Disorder (ASD) does not develop in adulthood, but symptoms can change for individuals over time because learning occurs. 

Symptoms of ASD come in all shapes and forms because this is a spectrum disorder and therefore people present somewhere on a spectrum, often referred to as from low functioning to high functioning. 

To answer the question directly, people who present symptoms later in life, as you say, often do present those symptoms in childhood but they aren't noticed by the adults in the child's life until it becomes a real problem for the individual. 

As an example, interpersonal skills are learned during childhood. It is perfectly acceptable for children to be at slightly different stages during development with regard to interpersonal skills. Some children will be more advanced then others. Interpersonal skills can be difficult for people with ASD to grasp and so someone with high functioning Autism can take longer to develop these skills, if they are developed at all. So that would potentially not show up until later when the other kids have developed these skills and the child with undiagnosed ASD has not.

Often, 30+ years ago, before we had more advanced treatments and assessments children would 'fall through the cracks' and be labeled as gifted, rather then as having high functioning ASD. This would explain why some people who are highly successful in adulthood are diagnosed in adulthood rather then childhood. People with high functioning ASD often have a savant type qualities, meaning they are very skilled in some specific area. 
The diagnostic criteria (DSM) has been amended over the years which would account for some adults being diagnosed now, as they may not have qualified before the DSM changed the criteria. 

This would be an interesting study to read to understand social interaction among children with high functioning ASD: Bauminger, N., Solomon, M., Aviezer, A. et al. J Abnorm Child Psychol (2008) 36: 135. https://doi.org/10.1007/s10802-007-9156-x

This article discusses changing symptoms (Behavioural) in youth and adults with ASD. 

This might be a good book for you to read together a good understanding of ASD in development, which might give you a better understanding as to why people can go undiagnosed until adulthood. Handbook of autism and pervasive developmental disorders, 2nd ed.

In conclusion, There are many reasons that someone might not be diagnosed until later in life, and autism is a spectrum which means people have many different symptoms that range on a vast scale. 



References 

Bauminger, N., Solomon, M., Aviezer, A. et al. J Abnorm Child Psychol (2008) 36: 135. https://doi.org/10.1007/s10802-007-9156-x

Shattuck, P.T., Seltzer, M.M., Greenberg, J.S. et al. J Autism Dev Disord (2007) 37: 1735. https://doi.org/10.1007/s10803-006-0307-7