Technically it is of course nine months old from conception, but it is new in the world without the protection of the womb, so it is new-born.
A baby is an infant that is not able to move on its own and is totally dependent on parents for care.
An infant is able to move around and is beginning to take independent decision.
A child is able to walk and talk, and carry out independent actions.
A teenager has reached the age of 10 [or 12 - depends on the individual view] years, and has not yet reached the age of 20 years.
An adolescent is pre-adult in maturation, going though all the hormonal changes needed for full preocreative status.
An adult is legally someone of voting age, over 18, or over the age of 21 years. They are fully mature sexually.
Middle-age is flexible. Depending on the possible full life term it can be around the age of 45+ years old, or around the age of 60+ years. Until recently someone over 65 was regarded as old; now they are middle-aged.
The trouble with trying to specify exact ages for all these terms is that individuals vary and there is no one specification for all. In your own world and culture, how would you regard each of these stages?
2. What is the difference of a newborn, baby & infant?
3. What is the guideline for the age of :
new born baby
The terms generally don't have distinct cut offs.
However, a teenager (teen), is someone in their teens, ie. between thirteen and nineteen, hence the name. "Teenager" might be loosely used in England for younger children if Anglika is right, but I haven't heard of it.
I use "baby" much more broadly than Anglika. I probably use it until "toddler" state, or around the one-year point.
On the other hand, I think of "infants" as the youngest of them all. In the US, if you go to a day-care center, the "infants" are the youngest - starting at six weeks (the youngest most day-care centers will take a child).
We also would not call a 10-year-old a teenager, but we have a term called a "tween" -- not yet a teen, but no longer a young child (they are inbetween, hence the name). The tweens are that 10-12 set.