• by Gavin O'Malley  @mp_gavin, February 28, 2014

Despite finally finding a mainstream audience, Twitter’s growing problems are not going away.

Next year, the company’s domestic growth will fall from nearly 20% in 2003 to below 10% -- and in four short years, growth will slow to just 6.4%, according to a new forecast from eMarketer. In other words, by 2018, Twitter’s usership will grow from about 43 million U.S. consumers in 2013 to 65 million -- or about half of Facebook’s current domestic user base.

Not unlike some other social networks, an aging audience is likely to blame for Twitter’s troubles.

“A maturing user base means that steep growth is in the rearview for Twitter in the U.S., which fuels marketplace concerns that Twitter is not growing fast enough on the heels of a recently launched IPO,” the research firm explains in a new report.

Among the demographics, 25- to 34-year-olds currently represent nearly 40% of Twitter’s total domestic user base. The problem: by 2018, the percentage of 35- to-44-year-olds who use Twitter (29.1%) will nearly match the percentage of teen users (29.7%).

Twitter’s adoption issues are no secret. At the beginning of the month, the company revealed that only 1 million U.S. consumers joined up during the fourth quarter of the year. Worse yet, Timeline views -- Twitter’s key ad engagement metric -- declined sequentially from 159 billion to 148 billion during the quarter.

On the bright side, “a well-established user base can also be a less volatile user base,” according to eMarketer, adding that older users are more receptive to advertising on Twitter. “Twitter’s maturing users, not only in numbers but also in age, could influence its advertising revenue potential.”