We singers are lucky. Our musical instrument is built into our bodies and came fully assembled and functional the day we were born. When we were babies, we were using it correctly. When we start to train our singing voice, we are basically training ourselves to use the instrument as it was intended.
It's not much different from our speaking voice. When we find the connection between the two it's not that difficult to develop your singing voice.
Your voice knows how to work properly. If it's not, you're probably making it work in a way that's not natural to it. This sometimes comes from misconceptions about training the voice. When you connect with it's natural function and abilities, you'll have a better understanding of it. You'll learn to trust it and treat it with the respect it deserves.
Some exercises (for example, those to warm up the voice) have specific intended results for all voices. So, we all do them, but we each have our favourites.
Some exercises are used to strengthen a certain ability, or correct a certain error. Because we all have different abilities and vocal faults, these exercises are specific to the singer.
Exercises that work for one singer might not work for you. Or they might work for you if you approach them differently. The key is to discover what you need to practice and how you practice.
Playfulness brings freedom when it comes to training the voice. I've learned myself, and see all the time with students, that if you give your voice the freedom to explore new exercises, concepts and ideas with a playful attitude, your voice will reward you with happy surprises about what it can do.
I've worked with singers of all ages, from early teens to people in their forties and fifties and I have always found that abilities and the learning curve match the attitude and not the age. So, don't worry if you're 45 and have never had a lesson. It doesn't mean you can't improve and if you have been singing all your life, you are not a "beginner".
Some of us are lucky enough to find a teacher who we connect well with and enjoy regular lessons. Regular lessons are productive for those of us with long-term goals which require consistent training of the voice, or want to explore their voice's potential or simply want to increase their repertoire.
Then there are those of us who prefer to go it alone and try to train ourselves. There are different reasons for this: time, money, accessibility or fear of incompatibility with the teacher's concepts and their training method. With a little bit of help and advice now and again, these singers can also progress nicely in their development.