That's why someone's financial situation should be considered before deciding to have kids. It is a conscious decision that is actively taken and therefore no reason to put younger people at a disadvantage.
> As for age discrimination: is only letting people vote at a certain age, age discrimination?
Yes, that is age discrimination. It is usually accepted, because there is an argument for it: in the public's view you should be mature enough to understand the importance of elections and inform yourself before you can vote. That maturity certainly depends on age to some degree. Nevertheless, since it is still age discrimination, the specific age is often debated and here in germany it was reduced multiple times in the past, for different elections. I have even seen arguments that said kids should have a sort of "proxy" vote through their parents, by deciding who to vote for together.