What musical instrument is easier for girls in their twenties to learn by themselves?

At the age of twenty, you can learn anything as long as you are willing to learn. Maybe some experts have something to say, such as studying early and studying late. That's an expert's private matter. We can listen to it, but don't believe it.

Musical instruments depend on your own preferences and personal conditions. As women, you can choose flute, saxophone, clarinet and horn for playing musical instruments, erhu and violin for playing stringed instruments, guitar, guzheng, pipa, zhongruan and Qin Yue for plucking instruments, dulcimer and drum kit for percussion instruments, piano and electronic organ for keyboard instruments, not to mention other relatively unpopular instruments.

As for the harmonica, I don't think it is a very good instrument. One reason is that the harmonica can only play slow-paced music, but it can't do anything about fast-paced music. Another reason is that playing the harmonica is unsanitary. It is actually very difficult to learn, requires a lot of vital capacity, and the volume is relatively low.