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
