There are some very extreme opinions out there regarding this topic. Is music something we need to introduce in childhood or else not even bother? Or do adults have an advantage over children, perhaps to the extent that it is cruel to enroll children in music lessons? Does age really matter at all?
Often, when prospective clients approach me about lessons, they are almost apologetic about their age or their child's age, as if they are preparing themselves for rejection. So, before I address this topic, I want to make something perfectly clear: music education is for everyone, from conception to death. There are very young children who probably shouldn't take instrumental music lessons or shouldn't study a particular instrument until they have the prerequisite strength, coordination, and size to manage it. We can't expect an infant to hold a violin when they can't even support their own bodies, yet. But otherwise, there is no such thing as being too young, and there certainly is no such thing as being too old.
There are definitely things that are easier to learn as a child than as an adult, not to mention impractical to put off. Walking, eating, speaking, reading. But does music education really fall in that category?
Children are less likely to hesitate to do a particular exercise that might make an adult feel silly. They have fewer responsibilities, which might make practice easier to schedule, provided parents help with this by accounting for practice when they make other plans.
Some instruments are only available in one size, which isn't manageable for small children. And there are reasons that each of those instruments has to be a certain size. A wind or brass instrument's range is determined by it's length, or more precisely the length of the vibrating column of air inside the instrument. Make it smaller and you also make it higher in pitch, in which case it's now a different instrument. Sometimes, you can add a curve in such an instrument to make it smaller. This is common with the flute. Curving the head joint brings the keys closer to the player, making it more comfortable for children to reach them. But in general, instrument choice is limited for a child in a way that it is not limited for an adult.
Theory is pretty much impossible for a child to fully understand. We can work around that by focusing more on technique. Children can be taught to read music at a very young age, and there are theory concepts they can memorize without needing to understand "why."
It's really hard for children to accept that music lessons aren't always fun. It takes a lot of work, and they might not want to do the work. This in turn puts parents in the position of deciding whether they should allow their child to quit or insist that they continue. Opinions on this are going to vary based on how much the parent values music education in the first place. If they were also primarily motivated by a desire for their child to have some fun, then walking away from lessons might be an easy choice for them. But if music education is just as important to them as academics, they won't permit their child to quit - ever.
While children have fewer responsibilities, adults have more control over their time and better time management skills. Adults have more freedom to make choices. It's just them and their private teacher, no parents or school orchestra teachers or youth orchestra directors or anyone else. It's hard to try to please multiple people as a student. Sometimes the pressure really gets to young musicians. There's something to be said for learning only because you want to learn, not because someone else is requiring it of you.
Also, adults tend to be more aware of the consequences of not practicing. If nothing else, they understand that they are spending their money on this, and therefore it should be a priority.
Younger hands might be more flexible, but older hands might have better coordination. Adults are more likely to hurt themselves or to ignore pain, but children might have more trouble with injury prevention because some of it is counter-intuitive.
In my opinion, the advantages and disadvantages of being a child balance out those of being an adult. There is no objective reason to say one demographic has it easier than the other.