Is it normal for inground pool liners do get wrinkles after only 5 years?  Is
it a sign that there is trouble ahead, i.e. leaks, have to patch or
get new liner?

Hi shellbum3535,

I grew up with an inground pool, my Dad groused about the wrinkles for
a couple days before he was satisfied. I remember he was on the phone
with the installer who told him how to get them out as it was filling.

Pool Liner Installation Instructions
http://www.websweeper.com/php/pool_liners/abg-002.php
..."Slight wrinkles appearing during the installation of a pool liner
is natural. This is caused by an over allowance of vinyl material to
avoid damage to the linear if shrinkage should occur.

Smooth out the floor of the liner by pushing it toward the pool wall.
A soft broom will be helpful.  Never forcefully pull or drag the
liner.

NOTE: The liner is not a structural part of the pool.  Its only
function is to keep the water from leaking out of the pool.  The water
pressure is held by the pool wall and structural parts, not by the
liner.  The liner must be installed properly with all the areas
touching the ground, earth mound, pool wall and not supporting the
weight of the water, which will damage the liner..."

And here's my original comment from below:

Wrinkles in Swimming Pool Liners 
http://www.faqfarm.com/Q/How_do_you_get_wrinkles_out_of_a_pool_liner


~~Cynthia


Search terms:
inground "pool liner" wrinkles

Perhaps not normal, but about average. Expect trouble in 5 to 10
years, perhaps sooner, if the pool is left empty for more than a day
or the ph is frequently out of the range 7.2 to 7.6   Neil

Wrinkles in Swimming Pool Liners 
http://www.faqfarm.com/Q/How_do_you_get_wrinkles_out_of_a_pool_liner