iPad doesn't turn on intermittently

My iPad Air 2 was gifted to me 3-4 years ago (now is Aug 2017). Over that time, normal use is every evening for about three hours. The past three months, it has, from time to time, not turned on. This has increased in frequency and now occurs every other day. When that happens, I press the power and home button until the apple logo comes on. This is not the result of a discharged battery; the iPad always has some sort of charge. Tonight, it took a looooong time to boot up and had 47% power when it came back on. Is something wearing out or is it related to battery?