I walked my first Camino in 2013, at age 59, and celebrated my 60th birthday at the noon Pilgrim Mass. They even trotted out the Botafumeiro...just for me! Hey, that's MY story and I am sticking to it! But, it WAS my 60th birthday, and they DID use the Botafumeiro. I was seeing it for the first time, so the association is logical.
Along the Way, on my first go, I met men and women of all ages walking solo and in groups...all the way from St. Jean Pied de Port to Santiago, and from all starting points in between. From age 7 (walking with parents) to age 75 (retired priest) walking with a friend, you will meet all sorts of folks.
In 2014, second Camino, and after Sarria, I even met a young couple with a newborn baby (less than 4 months old). The woman had planned and dreamed of walking the Camino for years, and she said that she was not about to let something like a new baby get in the way. So, the infant was carried in a hooded rucksack-like carrier, facing the mother or the father, in shifts. I later saw them in Santiago. So, no, 18 is definitely NOT too young to walk a Camino.
This year, while working as a volunteer at the Pilgrim Office, I met a South Korean family of four who had walked from St. Jean Pied de Port in France (775 km). The mom and dad were likely in their mid to late 30s. The son was 8 and the daughter 6. Yes, they walked the entire way! There was no carrier or stroller for the little girl. The boy had used a push scooter with skateboard wheels and a vertical handle - ALL THE WAY. He swapped off with his sister...
I gave them one of my red foam clown noses that I use to take group photos of pilgrims outside the office. They were thrilled and the parents relieved that the kids could entertain themselves. I customarily bring a dozen or so each year and give them away to young kids who are fascinated by them.
Given that your family and friends will naturally be concerned for you, PLEASE leave a contact plan with them. Tell them you will call home on a certain day, at a certain time (+/-) and STICK TO THAT PLAN. If you have a smart phone. Free Wi-Fi is commonly available in cafes, hostals, and many albergues.
I do NOT recommend using social media for this, unless you are writing a blog. I DO recommend simple text messages, e-mails, or perhaps a weekly phone call. Personally, I must call my mid-80s mother at 10:00 EDT in the US from wherever I am in the world every Sunday, without fail. it can be a nuisance. But to her it means everything. Simply set an alarm on your mobile and you will not forget.
About 99.99% of the times we read about "lost pilgrims" on the Forum, it is from the family of a pilgrim who failed to let their family know where they were and that they were fine. It is simple to do, and allays a lot of anxiety back home.
As it sounds, from your OP, that you will be in Spain, you should already have a SIM in your phone that provides voice, text, and data capabilities. USE THEM.
If you are a young woman, the easiest way to stay safe is to walk with or near others. After all, you are all headed the same direction...
As you stop each day, assess who walked around you that day. Meet people. Chat them up. Ask seemingly reliable and mature people (any age will do) if you can "tag along" with them tomorrow. You do not need to become their "best friend forever (BFF)." Just walk so they are within 50-100 meters or so, in front of or behind you. That, and carrying a loud police or referee type whistle on a cord, will certainly keep you safe.
The VERY few assaults that do occur, very infrequently, always occur when a pilgrim (usually a woman) walks solo, in a secluded area, without other pilgrims within visual or shouting distance. Other pilgrims will ALWAYS come to your aid if you whistle loudly or shout "HELP" or "AYUDA."
All of this said, please understand that walking a Camino is safer, much safer than being in most any large city on this planet, except maybe Singapore, which is really REALLY safe.
I hope this helps.