I am nowhere near you, but I start them at 3yo.
However
When we start at 3yo, we are very much not attempting to play 'proper tunes.' We use the piano as a vehicle for exploring musical concepts like high/low, loud/quiet, and work on recognising patterns, sign-to-sound recognition and counting skills (at 3yo it's not obvious if there are three or four sounds to play). We work on the ability to move just one finger independently. We mix in singing/dancing/gross motor movement for developing rhythm skills. I use a programme called Wunderkeys - the website is www.wunderkeys.com and there is a teacher search facility.
At 4yo (reception), depending on the child and his/her literacy, we are able to learn the white keys, begin reading 'proper' notes (first as rhythms with letter names, later on the stave) and to play tunes that only require a few fingers on each hand (using 1-2-3 eliminates the need to reach 1-2-3-4-5 in their correct position). At this age, I concentrate on good timing and a solid basic technique. I also insist on the children reading the music for themselves and try not to show them how to play things unless they get stuck. Most of my 4-5yo students play with a good legato (smoothly, without gaps between the notes) but not quite all of them have the motor skills necessary, and they tend to forget to apply it when playing actual tunes. We also play with teacher accompaniment so they get used to ensemble skills.
IOW, with young children, progress is usually slow. I refuse to teach children unless the child wants the lesson, and I always make this clear to the parents who usually appreciate that the slower pace is age-appropriate. We do lessons as long as the child enjoys them, and because the child enjoys them, not to do exams.