I think it’s more about how advanced a child already is at school rather than how much preparation they do or how early they start preparing for the 11+.
Our daughter’s CAT scores were always above 135, and her teacher consistently said she was about two years ahead in both english and maths. Her reading and creative writing skills like year 11 students. She loves creating writing and loves reading a lot! She was in independent all-through school, and honestly, we felt even if she doesn't get an offer from top schools she already has a place in seniors, so we didn’t put her under pressure. We never had tutor! While many others were taking extra Maths and English lessons, we chose to focus on LAMDA, art, music, and sports instead. Only in Year 6 did she did a few practice sessions just to get familiar with the exam format.
On the other hand, one of her friends at the same school had been taking extra maths and English lessons for two years, yet still didn’t receive offers from St Paul’s Girls School or City of London School for Girls. But my daughter did and she will start City of London Girls next year.
Every child is unique and different. I don’t think it’s necessarily about starting early or doing more. Only you can really tell whether it’s “too late,” because sometimes, no matter how early you start, it doesn’t guarantee a particular outcome and for some children its never too late!