When they are not going anywhere with an adult who is not less than 15 years old, children ages 5 to 14 must travel as Unaccompanied Minors. Only straight flights without an alteration of plane or flight number are acceptable for unaccompanied minors.
https://www.spritflight.com/spirit-airlines-minor-policy.html
https://www.spritflight.com/spirit-airlines-minor-policy.html