It is most concerning for any parent to become aware of or have differences in their children pointed out. Most often these are normal variations that are commonly seen and should be no cause for alarm. A single consultation with a clinical psychologist can allay concerns or if necessary, indicate simple strategies to improve the situation.
Greater attention is warranted when there are multiple indications or particularly where they are given by professionals. Again, a single consultation with a clinical psychologist can allay concerns or if necessary indicate ongoing strategies to improve the situation.
Where attention is urged is when children from 12 months seem unable to regulate their emotions and/or behaviour at an age appropriate level. This may include unusual levels of non-responsiveness, distress, rapid changes from distress to calm, apparent physical aggression and any behaviour which may be harmful such as head banging, biting and/or kicking that does not rapidly abate.
Lack of first words shortly after 12 months of age and gradual development of simple phrases by 18 months should attract attention as should resistance to being held by parents or primary carers, back arching, lack of responsiveness or unreasonable clinging.
Many early signs of later difficulties can be subtle. If you have any concerns, do not think of yourself as reactive. The best time to intervene is always AS EARLY AS POSSIBLE.