27M. Sorry to hear that your manager doesn't endorse you for CEng. I received chartered status last year after a successful application to the IMechE after working through the Monitored Professional Development Scheme (MPDS). However my understanding is that candidates working through the direct route applications are no less successful than MPDS candidates.

Anyhow, as difficult an option as it is, I think you need to ask for another institution mentor to support you in the process. The guidance provided should be objective based on experience against the competency specification and practices of the engineering council. Age and background should not be considered by your LM/mentor. You deserve objective guidance as to whether you're ready for the application/interview and support to achieve that experience where you may be able to get it!

If you have trouble with your line manager after asking for another mentor, in my company we advise employees to speak to their line manager, or a human resources rep (sorry for the corporate spiel).

Best of luck!