Does acne go away after it present itself in the age of 20? Or redness of skin?

Usually is outgrown. Acne is related to hormones that start to be made at puberty in boys and girls. The hormones stimulate the oil gland, sebaceous gland, to become large and active, to produce alot of oil and often a bad, thick kind of oil, sebum. The first acne are little clogges pores, whiteheads and black heads, buf often the acne gets bigger and inflammed over time. Many people end acne at age 20 but not all.
Acne at any age. Acne is a complex combination of diseases whose appearance is well described but etiology is unclear. There are a variety of types of acne that appear at almost any age including in neonates, classically in teenagers, and well into adulthood. Commonly these are associated with alterations in hormone status. Whereas mild cases may resolve with no sequelae, cystic acne may produce permanent scarring.
Can go away. Acne can go away with a proper care such as topical bezoyl peroxide cream and washing with a proper cleanser if it is mild.
Usually. Acne usually decreases in severity as you enter and go through your 20s. There may be pitting and scarring which never resolve completely. Redness of the skin should improve as the overall acne subsides. Scarring can be improved with various treatments, most of which can be performed in the office.
Most. Most acne resolves over time but you will find many people who know otherwise. Redness and scars can be permanent. New lasers are fair and can be helful. Don' believe all the promises though.