I would generally say since the money is needed more near term (a few years) you should be more conservative (more bonds/cd/cash, less/limited stocks).
For example Vanguards 2020 target retirement fund is 56/44 (stocks/bonds).
For me, given what the trust money is being used for and your role as trustee, I would be more conservative. Other considerations that come to mind are how much is in the fund and where the 16 year old is thinking about going, whether you want money left over and if grad school might be in their future.