As said above, graduate would be a lot longer and definitely a lot more expensive. You get the tuition fee paid for and a maintenance loan for your first degree, but for your second degree you can only get the maintenance loan (I believe), so paying 9.25k tuition fee for 4/5 years may be rather difficult while on a demanding course.
Above has explained well how work experience is working with Covid right now. In normal times, most universities will ask for a minimum of 2 weeks clinical (i.e. vet practice) and 2-4 weeks non-clinical (i.e. other animal environments like kennels, stables, farms) and they like you to have ideally had all 'types' of animals (smalls, equine and farm) but it is not a necessity. Again when things were normal I'd say it is best to try to get above the minimum amount to show commitment, dedication, have more to talk about in interviews/forms etc, but right now it is not that accessible so I wouldn't worry too much and do what you can.
As vet med is such a competitive degree, a large proportion of the students actually do not come straight from school, so at 21 you definitely wouldn't be in the minority and have seen many older people applying! I didn't get any offers last year and reapplied, so I am currently on a gap year and will be 20 when I start. I wouldn't let age hold you back - it's only one more year in the grand scheme of things and I'd say a risk worthwhile for the potential to get your dream career.