First off, reaching the reproductive maturity depends on many factors (biotic and abiotic). As a result it is difficult to talk about a specific age because stress can postpone the development of trees

However, as an order of magnitude I would say the following:

Bruguiera gymnorrhiza grows in brackish water and should flower at approximately 5 years of age.

Nypa fruticans also grows in brackish water and should flower from the age of 4.

Rhizophora mucronata is generally found in areas with a low salinity. It should be able to flower at the age of 3.

To be fair, most of the information I looked up myself, so I don’t know it by heart or experience. As such, be aware of possible errors.

I could not study the synecology of the mangrove species, but during my regular visits to the ecosystem I observed flowering of these species takes not less than 5 years period especially for the genera of Rhizophora and Bruguiera. Nypa may get the flowering after 3 years. As said by Jeroen some times environmental factors and hydrography play important role in the flowering 

In our natural experimental field at Jharkhali(In Indian Sundarbans) we have got flowering Bruguiera gymnorrhiza at 5 years (approx.). Other species of Bruguiera like B. sexangula, Bruguiera parviflora and Bruguiera cylindrica took much more time for maturation. another important thing that In Indian Sundarbans biological potantiality is much more in B. gymnorrhiza than other three.

Rhizophora mucronata took less than 6 years(rarely very few of them even matured before 4 years. Rhizophora epiculata takes much more time.

In  Nypa fruiticans we havn't got any flowering even at the age of seven. That is may be due to more salinity in our experimental field as the species is less salinity loving species.