If you have lipoprotein(a), Lp(a)--the most aggressive known cause of heart disease that no one has heard of--then you need to tell your children.
Lp(a) is a "cleanly" inherited genetic pattern: If either parent has it, there's a 50% chance that you have it. If you have it, then there's a 50% likelihood that each of your children has it. (Note that each child experiences a likelihood of 50%, not 50% of your children. This is because each child is conceived as an independent statistical event. So much for romance!)
The atherogenicity (plaque-causing potential) of Lp(a) also tends to get transmitted. In other words, if your Dad had a heart attack at age 50 due to Lp(a) and you share Lp(a), then you likely share a similar magnitude of risk as your Dad. If your Mom had Lp(a), though passed quietly at age 89 without any overt evidence of heart disease, then you are likely to share the relatively benign form of Lp(a).
For most of us with Lp(a), however, it is best to assume that it has at least some potential for causing heart disease, being the most aggressive cause known. (That is, until we have the ability in everyday clinical practice to characterize Lp(a) by assessing such factors as the size of the apoprotein(a) molecule, the number of kringle "repeats" on the tail, etc. Until then, we need to rely on the crude, though helpful, observation of family history.)
At what age should you inform your children? There's no hard-and-fast rule. However, I generally suggest to patients that they talk about Lp(a) with their children when they reach their 20s or 30s, old enough to begin to understand the implications and begin to think about adopting healthier lifestyles. Is treatment required at, say, age 35? That depends on the pattern of Lp(a)-related heart disease in the family: With exceptionally aggressive forms, it might be reasonable to begin treatment at this relatively early age.