In my opinion the biggest issue of a late ATC career start is the lifetime loss in salary pay. Where I'm employed you need 20 years seniority to reach the top level salary of ~155k USD/year. I will reach it at 39, while others I graduated with will reach it at 53. 25 vs 12 years at the top of the pay ladder is quite significant.
Training wise I see no problem with trainees in their 30s. With the right abilities (which you are tested and selected for) and the right attitude it's no problem. I started as an OJTI when I was 23, and I've never seen any issues with any trainee due to age. I acutually think I've encountered more 21 year olds than 35 year olds with the wrong attitude having their training terminated.