General:
* Development
* Growth
* Ripening
* Flowering
* Coming of age
* Evolution
* Progression
More specific:
* Puberty (for biological maturation)
* Adulthood (for reaching maturity)
* Refinement (for intellectual or artistic maturation)
* Aging (for natural biological maturation)
Figurative:
* Full bloom
* Prime
* Zenith
* Peak
The best synonym will depend on the specific context in which you are using the word "maturation." For example, if you are talking about the maturation of a fruit, "ripening" would be a good synonym. But if you are talking about the maturation of a person, "development" or "coming of age" might be better choices.
Can you give me more context about what you are writing about? This will help me give you the most relevant and accurate synonyms.