24 is the last age you are officially young. You have not yet reached the mid point of your twenties and so you can still legitimately be considered young.
25 is a no mans land age. You are neither young nor old.
26 is the beginning of oldness to which there are many stages. At 26 you can no longer be described as young as you have crossed over that mid-20s marker and definitely have 30 clear in your sights. You will have left any education long behind and be firmly on your way career-wise.
30 is the next milestone to be reached. However, just like 40 year olds today are considered to be the equivalent of 30 year olds in previous generations, 30 year olds are now the new 25 year olds. Being 30 is not quite the massive milestone it once was as although you are no longer in your 20s (which sucks big time), society does not expect or judge you necessarily to have progressed in areas such as career, marriage, family and home ownership like it used to. In 2011, being 33 and being single and renting your home is perfectly fine, whereas in 1961 it would have been different.
40 is the new 30 as we all know with the expectations of a 40 year old in terms of where they are in their life comparable to what they used to be for a 30 year old a couple of generations ago. When you reach your 40s you are starting that ‘middle age’. You are old, but you are not dead yet.
So is 50 the new 40? Yes, the logic carries through to being 50 compared to societal expectations of 40 year olds 40-50 years ago. The same applies to 60 being the new 50.
And is 70 the knew 60? I’m too young care.