What is the best age to become pregnant?

No one best age! Ideally when you're physically, mentally, and practically ready. Young women conceive more easily than older, but natural fertility is good through a woman's 20's to early 30's. Most are quite able to conceive up to 36-37, but may be slower or more difficult. After age 38 or so it's quite variable. Risk of birth defects goes up slowly to about 1-in-200 at age 35, and increases more rapidly after.
Fertility age. The highest fertility is approximately between the ages of 18-32. There is a decline in fertility over time because of natural loss of eggs from both ovaries. The chance of pregnancy decreases dramatically by mid forties.