if you get to a point where you just want to have a child but still haven't found the right guy, there's always the option to adopt. there are lots of children out there who would greatly benefit from everything you have to offer. and once you find the right guy, you can still have your own biological children later.
First of all, to say a biological "clock is ticking" is just a figure of speech that means a woman's desire to have a child before she reaches menopause. Technically, a woman's "clock is ticking" from the start of her very first period.
A woman can generally conceive a child right up until menopause. When you will reach menopause, no one can predict. There is a genetic factor, health factors, etc. The average age of menopause is around 50. But of course that means some women reach it earlier, some later.
Typically, it is late thirties in which pregnancy starts to become more difficult to achieve and supposedly are considered higher risk pregnancies (although that last point has recently been debated in the medical community.)
Personally, I'm the type of woman who says don't let a desire for children control your choice in men. Make sure you love him; if you settle down with a man you don't love just so you can have children you are shortchanging yourself, him AND the child/children you may have of a truly loving relationship. On the other hand, some women choose to leave men out of the picture entirely and just go to a clinic and get a sperm donation. That I don't totally understand. I completely support any women with children who gets herself out of an emotionally unhealthy relationship and becomes a single mother, but usually in that case you still have some additional support as a parent in the father. But to INTENTIONALLY put yourself in that difficult situation from the start by deliberately getting pregnant "fatherlessly" is a huge burden to deliberately take on.
I for one easily accept never having children as a possibility should the right man NOT come along in time. Doesn't bother me in the least, and I'm already 34 and still childless.