How Long Will It Take My 12-year Old God-daughter’s Second Molar To Fully Come In After It Has Erupted?

My 12-year old god-daughter is very self conscious about her smile, and wants to get Invisalign asap.
We have been told that all her secondary molars need to have erupted, and that this normally takes place between the ages of 11 and 13. We have not consulted a dentist/orthodontist yet, but want to get some sort of idea when she can start the process, as she is very anxious.

Orthodontics Is Breaking Us, Even With Insurance. How Does Getting A Second Dental Policy Work?

The forms we fill out usually let you fill in 2 plans. What kinds of things do I need to look for in a second plan, and does anyone have first-hand knowledge in the limitations of doing this? I’d appreciate hearing any opinions on this or other ways to try and save money on my two kids’ orthodontic work. Thanks.