You are limited to a total of 5 characters per account. To spawn as an adult, click the New Character card and select "I prefer to begin life as an adult." You will then be able to browse races available for selection either by clicking on their picture or selecting the race name from the dropdown menu. After filling out the details of your upcoming character, selecting "this is my chosen form" will spawn the character into the world instantly - no backstory is allowed, they appear suddenly automatically at age 18 mentally and physically. Adults will spawn in a location where random members (at least 3) of their race exists.
To spawn as a child, select "I would like to experience childhood". Then select their race and gender. After that, your account will be put into a line to be matched with upcoming parents. As soon as this happens, your character will be spawned into the location of those parents at the age of five. As a child character, you do not get to name yourself - your mother will name you after you spawn. You are required to make an adult for your first character.