
A Singer sells their music. A singer has more freedom of expression. A singer does not need to dance. A singer can be ugly.
An Idol sells an image and dream. They are restricted in dating, eating, and personality. They cater to the fantasy of fans. An Idol also must uphold a certain level of beauty and image of innocence and purity. They have to be able to sing and dance at the minimum level. Rapping if they are an all rounder. Idols must entertain and interact with fans. Their job does not end at singing and making music. Idols have a high level parasocial relationship with fans.
The difference between an Idol and a singer is evident in the definition of the word Idol (def: an object of worship).

It is very hard to do that because they will always be an Idol first and foremost. That is the image ingrained in the mind of fans as well as their peers.
And Actually most rappers, singers, and dancers look down on idols. They are not acknowledged as "true" singers, etc. Most have to prove themselves after going solo. And even then, they may always be associated with being an Idol. The most famous examples of these are when Suga and Rapmonster from BTS changed from undergound rappers to Idol trainees. They were viewed as not proffessional rappers anymore. They are skilled and talented but being an idol comes with stigma that you don't take your craft as seriously and rely on your face only. Another example is Lay Zhang from Exo. He was once on a singing TV Show and all the singers looked down on him until he proved himself. And tbh, most groups have famous members and less famous members. As a group they can make profit but as individuals, to be blunt, singing is not a viable career. Hence why you will see Idols going into acting, variety show, and singing and modeling. Some do continue making music but they don't have the same rapport as they would have as a group and that all comes down to company support, their own talent, and luck.
To address the longevity of an Idol career, there is not necessarily an age limit. You can see this with Idols like IU who is 30+ but still continue to sing and make music. However if we are being blunt, a idol probably peaks in their 20s. They are more youthful appearance wise and their physical body can keep up with the insane work schedule/dancing and performing better than a 30 year old. It is just like any sport professional out there who retire by 30s-40s due to injuries/body no longer performing the same.
I have realised something with these Idol mangas. Why are they always about Idol groups instead of Solo Idols because I know there are solo idols as well since some anime and manga as solo idols.