It's a punt at the best of times, in your mid thirties even more so. If you treat it as such and can afford to do it without risking your livelihood, then go for it. You'll find numerous examples on here of people who made it into airlines at that age and older, albeit few recent ones.
but for goodness sake, get yourself independently aptitude tested e.g. GAPAN.
That's the best bit of advice, aside from getting the medical.