You can only create one sim at the start of your experience with The Sims Mobile. Additionally, you cannot choose his/her age. You can only create an adult sim. To change the name of the sim you have to click on the icon marked with the red circle in the picture above.
If you want to change the sex of your sim, simply click on the appropriate symbol. These symbols are marked with a red circle in the picture above. Unlike in The Sims 4, there are no additional functions when it comes to changing the gender. However, you can change the sex of your sim at any given time.