Two answers spring to mind:
For "early in terms of age": the main disadvantages will likely be financial and time-based – that is, being less able to focus as intently on career or individual dreams, building up savings and investments, etc as you work together to forge a strong lasting relationship.
For "early in terms of number of prior relationships": the main disadvantages will likely be in terms of knowing what you want out of the relationship itself, and how well you and your partner are likely to create these outcomes together.
In practical terms, the younger you are, the more statistically likely both cases will be true. This isn't to say that all young lovers are doomed – it's clear to see that some people know from a very early age what they want from life, and also that some couples "just get lucky" – that, despite a lack of knowing what to look for in a compatible lifemate, they find one just the same.
For the rest of us, large dividends are likely to be paid back to us by:
- experiencing a wide variety of romantic relationships as early and often as possible, and
- holding off until later to marry until both partners are well established in their individual life plans – doing otherwise unnecessarily increases the chance of later resentment, or time-and-money-related stresses damaging the relationship in the long term.