I'm 34 years-old and I want to learn to play the violin. I had one lesson recently, and felt like I did pretty good considering I had never before held a violin, much less played one.
So, I'd like to hear from anybody that has picked up an instrument for the very first time much later than is typical, and how successful you were.
I've started piano and violin as an adult, together with my child, and made decent progress. Don't fall for the "kids learn better than adults" or "you can't learn when you're older" mentality -- it's simply not true. There is no magic age for learning to play music, and adults often progress much faster than kids, because the ones who try are generally better motivated as well as being more developed. I know this firsthand, as I have progressed much further than my kid in both instruments, can see that he has the same difficulties by large, and no magic ability to surpass them.
That said, there are some obvious and some subtle advantages to starting very young. As an adult, you should probably not aim to be a concert musician given that it's a competitive field and you'd be competing with others who have been doing it for decades already. A child who learns from a capable or learning parent has the advantage of a much enriched learning environment, and can pick up things with less practice time.
Learning piano is difficult because you have to coordinate at least two different musical lines between the two hands, and have to do more theory learning to be able to sight-read well, which is also harder because of the two parts involved.
Learning violin is still harder though, because you have to get the pitch just right, and maintain that even though your own hand and fingers may be stretched against that, while also maintaining perfect control of the bow through changes. Learning violin can be very frustrating for those reasons, so you need a good environment, self-motivation, and patience to get over that and develop.
A practice mute can help a lot for some of the environmental factors in violin. Piano is much easier in this respect -- you can get a digital one and practice that at all hours using headphones.
You can use a digital tuner to help you get started in learning the pitches, and accompaniment tools such as
Smart Music to learn pitch matching by ear. These are highly recommended, and I can see a lot of improvement in my kid's intonation due to accompaniment, but you also have to learn to be able to play in tune without these aids.