My 2 1/2 yo uses one but isn't entirely successful with it all the time. She does an okay job. She has been using a fork regularly since she was about 18 mos old and I don't mean a toddler fork a regular one.
My son is 17 months and using a spoon. He's not very skilled at it yet, but he needs the practice. He has been using it for several months now. My best advice would be to let them use it when they start reaching for it.
At the age that you teach them to use a spoon. Using utensiles isn't instinctual, so if you haven't taught your child how to use one then they're not going to know. "Teaching" can also be just as simple as them watching you use one as well.
I give them a spoon as soon as they start self-feeding. Usually, they pick up the spoon, then set it down and opt for fingers at first. Around 18 months old, mine have begun using the spoon for about half of their meal. A little after their 2nd birthday, they were more able to use the spoon most of the time.