Your Q appears to concentrate on the effect of these factors on ONSET of CHD.
High cholesterol is likely to bring forward any ischaemic changes i.e. they are likely to occur earlier than in a person with normal cholesterol level. Other factors that are relevant are:-
a) How high the level is: a cholesterol level of 6 or so mmol/L might play a part [in conjunction with other risk factors] to bring e.g. angina pectoris or even an MI occurring a few years earlier
WHEREAS:
a very high cholesterol as in familial hypercholesterolaemia can result in family members as well the patient dying of MI very young e,g, in their 30s or 40s.
b) The balance between HDL cholesterol and LDL + VLDL cholesterol will have a distinct bearing: HDL is protective against CHD, whereas high LDL is a risk factor.
[II] Smoking is one of the most powerful drivers of atherosclerosis and ischaemic heart disease - I have come across patients who smoked heavily who died of MI in their early thirties. You can google the mechanisms of this disastrous effect.
[III] A diet high in unsaturated fats is perhaps even beneficial, while one high in saturated fats is deleterious - certain foods tend to increase e.g. cholesterol levels and others to reduce them - again the effect on "onset" is difficult to isolate or quantify for a single risk factor since all the relevant factors act jointly and over years.
When a clinician takes a history from a patient who e.g. presents with chest pain suggestive of myocardial ischaemia or worse, all these 3 and other factors need to be asked about, and a global clinical decision made together with findings of physical examination and special investigations.
I hope this gives you points to investigate further.
M
BTW thanks for the rep on your other Q!