There is no fixed time and age to start learning classical music. While some inherit classical music from their parents, others develop a taste for it over time. The adults may feel that it is too late to start learning an instrument, but all they really have to do is find a person who can teach you. Pick an instrument and join a class that you feel suitable for yourself. Even if you are 40 and the rest of the class is filled with kids in their 20s, it does not have anything to do with music. As long as you are learning from a professional, you can learn any instrument you want. Even if you are not a born pianist, you can learn the technicalities and practice them every day to master your compositions.