Twitter began automatically hyperlinking hashtags in 2009.
The change took effect on July 2, 2009, when Twitter linked hashtags in tweets to search results for the tagged word. That converted the hashtag from plain text into a clickable way to follow a topic or conversation.
The convention itself was older than Twitter’s hyperlinking feature. Chris Messina proposed using the hash symbol for Twitter groupings in 2007, and users adopted the idea during events such as the San Diego forest fires. The term “hash tag” had appeared in print earlier that same year.
A common mix-up is confusing the creation of hashtags with Twitter’s technical support for them. Twitter users were already writing hashtags before 2009; 2009 is the year Twitter began automatically turning them into links. Twitter later introduced Trending Topics in 2010, making rapidly popular hashtags easier to discover.