Hi mark,
I don't think 27 is too old!! You should go for it,your age isn't going to get in the way lots of people retrain in careers.
I started at 17 (I'm 30 now!) I didn't know much about dentistry I just knew I enjoyed working with people and helping people the rest I learnt as I'd only ever had 1 other job before that so I needed to learn not just dentistry but working (in the grownup world!)
In fairness I think all jobs have their stresses but in dental nursing its the multi tasking and managing your surgery, patients, extra duties that's stressful trying to keep to time and nurses never just seem to nurse we are support for patients, (clinician!) P.A!! To clinician, receptionist, cleaner, as well as all the duties we have like stock ordering, sterilising, etc but working in a team is brilliant and the satisfaction of helping someone to get through their treatment makes you feel it was all worth while.
You can gain qualifications and build on experience in specialists areas if you choose to.
I wouldn't change what I do I'd much rather be chasing my tail all day than doing a job that's slower paced everyday is. Different your def never bored!!
Hope you find a position or course!