To check the age of a domain, you can use various online tools and services that provide domain information, including the registration date. Here are a few methods:

WHOIS Lookup:

WHOIS is a protocol used to query databases that store the registered users or assignees of a domain name or an IP address.
You can use online WHOIS lookup tools to check domain information. There are many websites that offer this service. Just search for "WHOIS lookup" and enter the domain name you want to check.
Domain Age Checker Websites:

Several online tools specifically provide domain age information. Websites like WHOIS.net, DomainTools, and others offer services to check the registration date of a domain.
Domain Registrar:

Visit the website of the domain registrar where the domain is registered. Most registrars provide a domain management interface that includes information about the domain, including its registration date.

If your domain authority has not improved despite an increase in the number of links, it's essential to assess the quality and relevance of these links. Ensure that the new links are from reputable and contextually relevant sources, avoiding low-quality or spammy links. Additionally, consider the overall quality and relevance of your website content, as this plays a crucial role in influencing domain authority.