Not at all.
My now 5 year old is August born.
Back in 2020 he turned 3 in August and started school nursery in September for half days every day.
In 2021 he turned 4 in August and started reception in September for full days.
In 2022 he turned 5 in august then began year 1.
We now know he is autistic, but back in 2020 we didn't. He had severely delayed speech, very poor social skills, couldn't even tolerate other children being in the vicinity. He could only say about 5 single words and mostly communicated by crying, making noises and pointing. He couldn't recognise colours, letters, numbers, shapes, he had no basic learning abilities at all. At times we considered whether a mainstream education would even be possible.
Now, he is speaking in full clear sentences, he not only tolerates children he plays with them and has formed friendships, he can count, add, subtract, he can read basic phonics books, he knows all phonics sounds and can blend them. He is literally defying all the odds.
I Imagine we would be in a very different place now had we deferred for a year.
I know our situation is different due to autism but it just goes to show what a difference it can make to a child being in a school environment.
Someone is always going to be the youngest, but being the youngest makes no difference to their individual abilities.