I don't think it's too young at all, as long as they're actually enjoying it and want to do it. I don't really see why you'd need to motivate them to do it if they were actually interested in it. I didn't start til I was about 13 and had a succession of awful teachers, one of whom actually told me he hated classical music! He was also a kayboard teacher, so I taught most of the stuff myself because I loved playing. And even after all of that I went on to study at a conservatoire.
My son is currently 2 and we 'play' music together. He taps keys and I play. I'm expecting to start teaching him as soon as his fingers grow a little bit longer
I don't think it's too young at all. If you wnt to find a good teacher, a site musicteachers.co.uk has a searchable UK database plus tips of what you should ask the teacher and check before you arrange a lesson. Good luck!
I have taught 6 year olds who were too young to play piano and 6 year olds who are musically as bright as a button, highly motivated and achieving excellent progress.
Much depends on the individual child's personality and maturity, the home work/play ethic, the parents' willingness to be with their child at daily practice time and to show active interest in their piano lessons.
Go to the very best teacher you can afford, who enjoys teaching children.
The keys to success are wanting to play, effective practice routines, the right parental support and above all the right teacher.