Paper documents experience severe acidification and embrittlement. Nanocellulose is an excellent reinforcement material for paper documents owing to its compatibility and excellent mechanical strength. However, little research has been conducted on the aging resistance of nanocellulose-reinforced paper. In this study, six types of nanocelluloses were used to reinforce aged paper. The reinforcement and anti-aging performances were evaluated, and the anti-aging mechanism was further clarified. Nanocellulose with a high degree of polymerization can better enhance aged paper, and non-chemical nanocellulose also shows better anti-aging performance, such as nanocellulose prepared by mechanical or biological methods. However, nanocellulose prepared using chemical methods exhibits poor reinforcement and anti-aging performance. This is because it has a small particle size that is not beneficial for physical crosslinking with paper fibers. More importantly, the introduction of acidic or oxidizing groups on nanocellulose accelerates the acid hydrolysis and oxidation rate of paper fibers, especially nanocellulose prepared by 2,2,6,6-tetramethylpiperidine-1-oxyl oxidation, which should not be used to protect paper documents.