What is the best age to start thinking about getting pregnant?

No easy answer. A woman with regular menses has a high likelihood of becoming pregnant within 6 cycles with regular unprotected vaginal intercourse. However, a number of other factors might need consideration to choose the appropriate time. General health, educational or work aspirations, finiancial stability and emotional readiness to settle down all should be considered. Take the time to assess your situation.
Depends. First you should take prenatal vitamins before pregnancy to decrease risk of birth defects, consider the expense of children, time required, and support needed to care for kids. If you have too much money, too much time, too good of a relationship, and you're ready to complicate it, early 20's is physically a good age for pregnancy.
Before age 35. When you are physically and emotionally able to care for a child is the right time. Preferably before the age of 35, as women over 35 are at an increased risk for down syndrome, but those are the guidelines. You should take a multivitamin containing Folic Acid prior to conception.
No perfect age. When to try to get pregnant is a tough question to answer, and for every person that answer is different. Most people are physically able to get pregnant long before they are ready to care for a child. As people age fertility decreases, but I have patients having babies well into their 40's.