I am 19 but still cannot apply soap on back properly when bathing. This is a hectic work in order to clean my body i have to wash my back properly too. So is there any lifehack to apply soap easily?
Use a Japanese shower cloth. They're long enough to hold with a hand above and below the back and scrub the whole thing. You can move your hands around and arch your back to scrub different parts, but you'll get everything, and better, you'll know you got everything.
As a bonus, you can use it as your showering cloth for your whole body, so you're not storing an extra thing in your shower
You have several options when it comes to back cleaning.
My preferred method is to lather up my hair and let the suds drip down my back while rinsing. I find I'm able to get pretty full coverage on my back with soap, and as long as I haven't been shirtless, this light stripping of oils is sufficient.
Similarly, soaking in a soapy bath may also prove sufficient to clean off an accumulation of skin oils.
However, if your back is particularly dirty you may need to employ some tools to scrub it. As BrettFromLA suggests, the ideal tool is called a back brush.
Sans a back brush you could try draping washcloth over your shoulder and garbing the opposing ends with your hands in order to reach the areas you are otherwise unable to reach.