I think the issue isn't your age objectively as you say, you potentially would have 20 years in a new career. It's not uncommon for any profession to have late entrants like medicine, vet science, nursing, law, accountancy, as a second career.
I think the issue is likely to be energy levels and enthusiasm (or youthful ambitious turbo-power which is another way of putting it).
I work in a profession and any profession is very hierachical because people who have more experience, have seen more and know more. An average person + a lot of experience = a good person. The very best people + a lot of experience = a superlative guru.
This results in the junior people in most of these professsions being 'looked down on' and expected to do a lot of tough working - long hours, unsociable, not that interesting - as it is this labour that keeps the machines grinding.
When you are young and stupid some might say, you have a number of big energy drivers that are likely to be lacking as you get older -
*one is the basic desire to qualify and get a degree. You have to press on as this is where you are and you need to graduate.
*you are still used to education as a school leaver and university entrant. Taking exams is a skill that once you get out of its hard to get back into. Its not impossible just harder that it was when you are in the groove of it.
*the next is the basic desire to get a first job to progress your ambitions. This feels different at 25 than it will do at 45. In one sense you have more options, because you can take a low paid job because it is of interest to you or you feel you'd benefit long term career wise as it's under the wing of Mr Super Professor.
*Next is still being used to a hard student grind so it doesnt feel so bad. If you are 45 and have had years of other working life experience, maybe have other financial demands what you are able to accept or willing to accept or capable of tolerating with good grace and energy
*Finally there is the basic human problem of finding it harder to deal with the (sorry to say) basic lack of respect in the way juniors in professions tend to be treated simply because they are junior and lack experience. I've seen this and people who had first careers where they were really senior sometimes can really struggle with this. It counts for nothing in the eyes of the profession. It really doesn't matter if you once ran Tescos globally if you need to cut a piece of bone out of a skull under the eye of the senior surgeon.
If you are keen 100% go for it. It is totally achievable and people do do it but you will need to be confident you have enough drive and determination to power through the tough aspects.