Originally Posted by Red Herring
It is usually recommended to start early if you want to learn an instrument, but it's never too late.
Have you been studying an instrument as an adult?
Yes: I play drums, bass, keyboards and guitar.
If so which one and did you teach yourself or did you take lessons?
I took lessons for some of it, but I also taugh myself most of what I know.
Was there an instrument that you already knew how to play and that helped you with a new one later on?
Piano. It's the most basic for learning music theory.
How would you weigh theory vs practice?
Practice is everything. Having an intellectual understanding means nothing without the learned ability to play what you know. This takes lots of repetition, but is it's own fun.
Anecdotes and recommendations welcome!
You will get frustrated with the slow pace of it, but this is only a matter of perspective. Learn to love the sound and motion of playing and be patient with yourself. The only way to learn things quickly is to have played many hours at slow to moderate speeds, and playing things accurately at those speeds. Only then will rapid learning be possible.
My father took piano lessons at age 58, and was suprised that it was difficult to do (he had two PHD's)
Let's see if you guys can motivate me to pick up that neglected mandolin and give it another try
It's not unlike going from couch-potato to athlete. Possible, but very difficult at first. So be gentle with yourself, don't push hard and injure yourself, don't go so easy on yourself that you don't progress. Keep you hands, wrists and arms as relaxed as possible. Take some lessons and learn 'proper' technique. This is important. the established ways are around for a reason, so don't try to reinvent the wheel. It causes frustration later on when you realized that you have to relearn the basics in order to progress.
But you can do it!