Well, I'd vote for you counting birds you definitely saw in your life (it's your life list) as long as there is no doubt over the id. You don't necessarily magically 'become' a birder at some arbitrary point in time ... So if you have a clear memory of something iconic and memorable it would count, imo, for something less memorable and common which you have since seen (like Robins and Blue Tits?) it doesn't matter so much.So I have mused for a few months! I have gone for June 2017 as my start date, as that is when I first started writing down birds with confirmed ID. Annoyingly this misses a short-eared owl seen at the Roaches before I started birding seriously and somewhat less so the golden eagles I used to see on family holidays to Arran, amongst others! Also annoyingly for the first couple of years I did not write down the exact date sighted, I added them to a place list - all that changed on 15th August 2019 when I saw spoonbills, Buff-breasted sandpiper, long-billed dowitcher and black-necked grebe - what a day! I have collated all my sightings which, having discounted the black swan and pheasant, gives me a list of 137. I think I am going for the format pictured, but again am having another muse!
(Others may differ in opinion )