I think it's a great idea, if you really want a kid.
You're in Brasil, correct? (Stalking your profile for this answer) I have relatives in Brasil that have adopted older children. Young kids are more "popular" there because of some myth that a child can't think of a new person as a parental figure past the age of four or something. Maybe an older kid is a wise choice.
I don't think it's a very good idea. Every child deserves a father AND a mother. I know you have good intentions, but it's just not fair for the child to grow up without a mother. Especially if you adopt a girl. If you really are longing for kids, a good thing to do would be to temporarily foster some. But most of the kids in foster care have been through traumatic experiences or are special needs. Make sure you have enough time, patience, and love for them. You probably don't feel like marrying right now because you just haven't met the right girl. Don't worry, you're still young, you have plenty of time.
It's still better to have just one parent to look after you than none at all. Plenty of widows and widowers do an awesome job of raising their kids alone. Many kids in orphanages are already missing both of their parents, why not give them the next best thing?
Every child deserving a home and a provider of any sort is more important than having a opposite-sex double parent. All evidence points to children of single parents doing far better than children raised in the foster care or adoption systems. Furthermore there's no evidence that, if gender roles are important at all, can't be taught by other family members, friends and their family and teachers.
Ok so I'm a guy, got a good family (parents, brother, etc.), enough money and no desire for marriage but with a desire to be a father.
Is it a good idea for a single male man to adopt a child? Why?