Henry Clay was actually too young, when he was first appointed to the US senate. To my knowledge it didn't illicit any comment at the time. I'm not aware of anyone being denied a seat in the Early Republic due to their age, but I can think of one who was denied based on immigrant status. Albert Gallatin was elected senator from Pennsylvania in 1793, his election was challenged by a group of political opponents who claimed that he had not been in the United States for the required amount of time. The issue went before a senate committee and then the entire senate which decided on a party line vote not to seat him. The entire issue was almost assuredly political in nature, Gallatin (a swiss immigrant) had been in the United States for more than the nine years required. Gallatin would subsequently have an extremely successful political career, leading the Republicans in the house after Madison's departure, becoming the longest serving secretary of the treasury in US history, and helping negotiate the Treaty of Ghent. For all of his successes Gallatin's immigrant status would continue to hinder his political career throughout the rest of his life.