My boy is 6 months and still not sitting up.
My son didn't sit up until he was 7 months old. Now he does it all the time with no problem. Every baby moves at their own pace. It's definitely okay for them to move faster or slower than the 'typical' time frame. Don't compare to other babies either! Your baby is unique and do milestones in his/her own good time! Lucas has been a little 'slow' with things, including sitting up, but he's his own person! He'll do what he wants when he's ready and so will your baby. :)