Starting PhD at the age of 25 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. 


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*Also can one get a PhD in theoretical physics/mathematics in less than 5 years? 

*I have made this question community wiki. 

 A: I can't write it as a comment (i do not have enough reputation)According to wikipedia Preda Mihailescu received his phd at the age of 42 and then he proved Catalan's conjecture http://en.wikipedia.org/wiki/Preda_Mihailescu .So ,i do not think that it is really a matter of age...
A: 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.  
A: You are only three years older than those 22 year olds. Three years is nothing! I'm having difficulty understanding why you are extremely depressed about this. I have several friends who entered graduate school at age 25-27; it isn't that uncommon.
How long it will take for you to get your PhD will depend on a number of factors, one of which is the field you would like to work in. Some fields, such as number theory, are notoriously difficult to get into, and it may take longer to arrive at research-level mathematics in those fields (relative to newer or less-established fields). If you are concerned about getting in and getting out quickly you might want to steer yourself towards fields that you feel you could make an earlier contribution to; but really, I think that you are unreasonably worried about your position and that you should just take a deep breath.
In the American system it is possible to get the PhD in 4 years but I believe that 5 years is more common, and some take longer. In contrast, 4 years is the normal time frame in the British and Australian systems.
A: Here's a technical point that is tangentially relevant for your question.
Some grant-giving institutions have, among their list of official requirements things like:
"The applicant should have received his/her PhD no earlier than two year ago, and no later than 7 years ago".
As far as applying for those kind of grants is concerned, it is an advantage to have finished your PhD later.
A: 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
A: 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.
A: I guess this is not the kind of example you are after, but I couldn’t resist: Brian May has finished his PhD in astrophysics when he was 60.
A: One of the most highly respected researchers in my own field (category theory and its applications) obtained his PhD at an age close to 40. I really think you shouldn't worry. Dig deep and do good mathematics. 
A: I believe (I am sure someone will correct me if I am wrong) that the vast majority of Israeli mathematicians start their PhDs close to that age. The Israeli school is extremely strong (however, as my Israeli friends point out, that might be the reason for why they concentrate in combinatorial fields -- much less baggage than in, say, Algebraic Geometry).
And to add to the list of examples above, Joan Birman started her degree after raising her kids at a time were women did not had a pretty hard time, did very well afterwards, and is very active today (and she is even older than the OP)
