TITLE: Starting PhD at the age of 25
QUESTION [10 upvotes]: For over an year I have otherwise been pretty active on MathOverflow but for this question I would like to remain anonymous. 
I would be starting my grad school in Fall 2011 in the US while in the 25th year of my life. I will be joining a grad school which is ranked by most lists within their top 10. 
I have been working and hope to continue to work in areas in theoretical physics which have a strong interface with mathematics, especially geometry. 
I understand that most people start their PhD at the age of 22 (or even below!). 
I would like to know how does it affect my career, now that I will be getting my PhD around the age of 30. I am very worried and extremely depressed that this is possibly too late to start a PhD. I guess most scientists become faculty by the age of 30 when I would be getting my PhD!
I would be happy to get any feedback/advice about starting PhD so late in life. 

Also can one get a PhD in theoretical physics/mathematics in less than 5 years? 
I have made this question community wiki.

REPLY [5 votes]: The difference between 25 and 22 may seem like a lot at your age, but honestly it's nothing in the context of a life span of 70 years or more.  You're certainly not too old to start graduate study and have a very productive career.

REPLY [2 votes]: Something that can be very satisfying is to acheive a series of long-term goals you have set.  Something that can be unsettling is the sense of achieving something unintended or finding that what was previously considered a good goal is not.
The issue of starting a Ph.D. at age 25 versus 22 or some other age is less important (I think) than the issue of whether this fits in with your other personal goals.  For example, I can't say that starting a Ph.D. at 25 kept me from starting a family over a decade later, but it probably contributed to the delay.  Hindsight shows me the advantages of starting a family earlier than later.  There are also issues of employment/financial support, degree to which you spend time not doing academics, etc.  If you think you have done a full self evaluation and found that Ph.D. studies fit, good for you; my bet is that you will need to reevaluate your goals on a yearly (if not more frequent) basis.
I do not want to discourage you from a Ph.D.  I think that a few hours (days, weeks) spent
doing serious thinking and planning now will ensure years of satisfaction to come.
Gerhard "Ask Me About System Design" Paseman, 2011.03.29

REPLY [2 votes]: In France most of students (in pure mathematics) start their PhD at the age 24. Indeed they get a A level at 18, then they need to study during 5 years  to get a master. They could start a PhD just after the master but most of the time they take one year more to pass the "agregation" diplom (a diplom for teaching). So you get 18+5+1=24.