aquinas, 1 year old can mean anywhere between 12 months and 23 months - and although I don't know what type of trampoline is at this school, we have allowed our DD in bouncy houses and on a gym trampoline when she was "1 year old" - when she was 20+ months and holding her own with her 3.6 year old brother. So it is really a matter of the child and the environment (we pulled DD out when the bigger kids came, but if it were just the 2-4 year old crowd and not too many kids, she had no trouble knowing where she was in space in relation to the other kids - but being hit in the face intentionally is a whole different ball game).

Regardless of the rationale and history, as other posters mentioned, it is critical that this behavior is addressed especially since you make it clear that this is usually targeted towards younger kids (I assume usually meaning smaller children). We are currently dealing with the reverse situation - a child twice DS's age who has shown some angry and inappropriate behavior towards DS and his classmates and usually when it involves a specific shared resource.

And I do find it somewhat sad - and worried - that he is still holding tight to the memories of when he was wronged - because if he does not learn to put those experiences into perspective, it only gets worse (don't ask me how I know - I have one sibling whose inability to let go of childhood incidents has disrupted our family relationships and her anger spilled over onto our children). I don't know that 4 years old is the age they can learn to put past into perspective, but it should not be brushed under the carpet or used as an excuse to allow his current behavior.