All the cells in our bodies get replaced quite frequently, but our brain cells are particularly hardy! In fact someone else asked me how long brain cells live for, and this was my answer:
For a long time it was thought that your brain cells pretty much ‘live’ as long as you do! You’re born with nearly all the brain cells you’ll ever have. Noe research seems to suggest that brain cells are replaced every 7 years or so.
That said, I don’t think the replacement happens in every area of the brain – some brain cells you’ll have all your life, I think.
But, sometimes we can lose brain cells that don’t get replaced, and all sorts of things can cause this, from a bump on the head to drug and alcohol abuse. But the most common cause of brain cells being destroyed and not being replaced is old age! There are also diseases called ‘neurodegenerative’ diseases, such as Alzheimer’s and motor neuron disease, which speed up this rate of decline.
I hope this answers your question, let me know if I’ve misunderstood it!