While deciding to buy a bicycle, you need to consider few parameters such as age, gender, price, quality, the purpose of buying, type of bicycle/features etc. For ex: if you are a boy of age 24, then you would consider to buy a bicycle with height of 18" inch, with gear & shocker, etc.
And if you are a girl of age 6 yrs, then you would consider a kid friendly bicycle with good safety features that the kid can ride.