cake DD started learning to read in English (with me) at 4 - of course she learnt more slowly, as she was 4... I do think if you wait til 6/ 7 they learn quicker, but that doesn't make either starting age intrinsically better. I never encouraged her to read in German before she started school at all, deliberately and because I didn't want to teach anything incorrectly in German, but she has picked German reading up really easily and her teacher raised no negative points at all on her first class report despite her being the youngest child in the class - she started a week before her 6th birthday, and has now started 2nd class but won't turn 7 til next week.
On the other hand Ds1 will turn 5 next week and has absolutely no interest in learning to read - he can write his own name and he can recognise the names of several of his friends, and of both his siblings, and few letters, but I have offered to teach him to read and he says not yet. He would have just started reception in the UK and would be one of the oldest in the class, but I am quite glad he has more time and doesn't have to start school yet. He is clever enough (he recognises all his numbers and can count up to 100 and can add) but he just isn't interested.
I think writing is a more problematic thing than reading actually if children start before they are ready - DD could draw exceptionally well at 3 - she was drawing people with bodies and detailed clothing and shoes, distinctive hair, all their features etc. at only just 3, and I look at pictures she did then, and there is one on the wall she copied from our wedding photo and it is still streets ahead of what DS1 could do now at nearly 5, his pictures are like DD's at about 2.5 and it's all down to pen control I think - although he holds his pencil correctly he just doesn't have the fine motor skills, so again I think he would find it frustrating being forced to learn to write more than his name on a daily basis).
DS1 and Dd are both late Sept birthdays but we let DD start school at nearly 6, whereas we have already decided to keep DS1 back til he is nearly 7 (we have told Kindergarten not to put him into the Vorschule group for small group activities so he will hopefully never realise he has been held back) - he still won't be the oldest in the class, there are kids up to 18 months older than DD in her class and nothing negative is said about it.
I am aware it can be the wrong decision either way though, starting too early or too late can be problematic, looking at the example of DH's brother who was doing his other brother's homework at age 3, but in those days wasn't allowed to start school til age 7 and was terribly bored and never engaged with school at all, and ended up leaving after 9th class, dropping out of his apprenticeship, and has done various manual jobs since, though he has just recently (age 26) gone back on a scheme to finish the apprenticeship... Could be nothing to do with when he started school and just his personality though!