Certainly children learn more quickly than adults, particularly when it comes to languages, and to skills. (That is, "proficiency, facility, or dexterity that is acquired or developed through training or experience.")
As a former US Figure Skating Basic Skills instructor, I observed this effect time and again when teaching school-age children as compared with teaching adults. (It's a skill that requires both physical learning and mental understanding, and practice, similar to musicianship.)
And it is well-documented that as we progress through our 30's, 40's, 50's, and upward, our learning rate decreases. (why-learning-is-harder-as-we-get-older)
However, there is absolutely no reason to avoid learning a new instrument in your 20's. In my own personal experience, I started learning "pop" organ at age 11, and studied viola from ages 15 through 18. Then -- having taken no musical lessons of any kind for 30 years -- I started studying piano at age 48, and found it enjoyable and not terribly difficult.
(Although piano and organ are both keyboard instruments, the left hand accompaniment
is entirely different, as is the expression. The organ has bass pedals, which the
piano doesn't. The organ has one or more expression pedals or "swell shoes" which
the piano doesn't. The piano's loudness and timbre are sensitive to touch, which
doesn't happen on the organ. They are different instruments.)
In both my experience teaching skills to others (figure skating and a couple of other skills), and in my experience as a music student, learning as an adult is
different than learning as a child, but not necessarily worse:
As a child, you may have more time available to practice, though this is not a hard-and-fast rule.
As an adult, you are likely to be more focused and more motivated in your practice and your study. You are better able to understand the need for boring things such as drills, and better able to master your own feelings and attitudes. Adults "buckle down" better than children do.
As a child, you do learn more quickly.
As an adult, you may tend to value what you're learning more, and may retain it better.
As a child, you approach things with fewer preconceived notions, and are likelier to accept things at face value. (I saw this all the time teaching children to skate, particularly when beginning to teach jumps.)
As an adult, you are more capable of analysis: both of what the teacher is saying and of what you yourself are doing. Some examples:
Teaching adults proper skating posture was far easier than teaching children. I could draw word pictures involving imaginary lines through parts of the body, and the adults could translate that into actions and posture. I had to physically bend the children into the shapes I wanted -- repeatedly -- before they could successfully reproduce the instructions. Then they would revert and have to be taught again, whereas adults only required a simple verbal reminder.
Learning aspects of musical theory was far easier for me as an adult than as a child. As a child, I understood that an E-flat was the same as a D-sharp, so "who cares what you call it." As an adult, I could see why the key of the composition affected the names (or identities) of the notes.
As an adult musical student, I'm able to analyze my own playing and state "___ is the right way to finger this, but my pinky finger on my right hand will always be weak due to a childhood injury, so I'm deliberately [incorrectly] doubling it up with my right ring finger so I'm able to strike a high note with particular force when needed."
As an adult, you are better capable of evaluating whether a particular teacher/instructor is the "right" one for you.
As an adult, you have much better proprioception than you did as child or even as a teen. For example, you should be able to learn the distances between frets (and strings) without looking, and learn it more quickly. As an adult, you've also done most of your growing, and your fingers are capable of being stronger than they were as a teen. If you are like me, you are also better able to ignore pain while developing calluses for playing.
The size of your hands may be another matter entirely. You may need to consider getting a 3/4 or 2/4 guitar, or switching to something smaller, such as the mandolin.
Not everyone is suited for every instrument. For example, I tried playing saxophone in 6th and 7th grades. I had trouble controlling my breathing, and I never could really learn the fingering well enough: there's no real pattern to it (it's dictated by physics). So, while I love listening to the sax, it's not the instrument for me.
I hope these observations serve to encourage you to continue your musical education. Learn any instrument you want, at any age you want!
