This is definitely not true. Adults have more trouble learning some kinds of new motor skills up to expert level, and if the physical skill requires e.g. extreme flexibility kids sometimes have a less fixed bone structure, but adults are a lot faster at getting to basic competence in new physical tasks by building from past experience, and for any other kind of skill adults can learn much faster than kids.
The real problem adults have vs. kids is available time to spend thinking about things (because they have other responsibilities), not raw learning speed.