Is it time to upgrade modem?

Ive had an actiontech modem model gt704wgb for 5-6 years.  Im on a dsl plan that alows download speeds up to 15mbps.  Ive been speed testing a lot for the last couple of months, and I generally get speeds around 5mbps.  I sometimes get lower, and a little higher.  Ive called customer service a handful of times and they recently said they optimised the line, because they noticed some problems.  Im still getting slow speeds though.  The say everything looks fine with m signal and everything.  Do you think my problem be with my modem, because it is old, and technology might have surpass its capabilitys?    

Re: Is it time to upgrade modem?
Contributor - Level 2
Is your plan "1.1-15" or "7.1-15" ? Up to means just that - UP TO 15.

The pricing plans are the same for anything over 1 Mbps.