i agree with sabbbi, it does change as we get older ourselves, but in reality people are living a lot longer these days so putting a number on it is quite difficult because it will be different for everyone depending on their health.
For e.g
my mam and dad are both 67 years old, my mam is a picture of health, very smart, hair done, still very active and always on the go, she hasnt got a single wrinkle on her face but suffers with usual ailments that accompany getting older.
My dad on the other hand hasnt been retired a yr yet and his health has plumited, he looks like a little old man, struggles to do certain things but his pride wont allow him to give in, its really sad to see him that way.
anyway, sorry i waffled on but if i had to put an age limited to the start of being old id say from around 65 xxx