When our children have tantrums, it's usually not fun to endure as the parent! Often the "terrible twos" begin around 18 months of age as the child starts to gain his/her independence. However, some children don't start this resistive behavior until they are three years old. Not every child is the same! When a child with a developmental delay or disability begins to receive intervention (therapy, counseling, ABA), it is not unusual to see the child have tantrums once the cognitive and language skills improve... they are going through a stage that they didn't go through at an age-expected time. So, sometimes children seem worse when really they are improving with cognitive reasoning.
Often, a child with a disability is tantrumming because he/she is: not able to comprehend what is asked of him/her, perceives the activity too difficult, is frustrated when needing help to complete an activity, is tired (fatigues quickly with weak muscles, poor sleep patterns, etc.), or can't fully express what he/she is thinking.
Here is a link that describes tantrums for children who aren't delayed. So a child who is delayed may not go through the behaviors described at that specific age, but the description of why they are acting that way still stands.
The Terrible Twos - Toddler Temper Tantrums - Parenting.com
I was fairly lucky with my daughter (now 8) having minimal tantrums... at the time I didn't think that though! My son has far surpassed her in the tantrum department! Many of his "fits" wouldn't occur if he was an only child and didn't have to share toys and my time. Hopefully, in the long run he will be better at sharing, waiting, and negotiating.