These loans could not be deemed excess benefit transactions unless the teenage borrowers are somehow considered “disqualified persons” with respect to the charity.  Disqualified persons are insiders such as officers, directors and those in a position to exercise substantial influence over the organization in the prior five years, plus certain family members and related businesses. (See Ready Reference Page: “Charities Must Avoid Excess Benefit Transactions”) 

Assuming that the borrowers are not disqualified persons, you still have to ask whether the loans constitute a private benefit that could cause the charity to lose its exempt status.  (See Ready Reference Page: “Charities May Not Confer Private Benefits”) But both the excess benefit rules and the private benefit rules have exceptions when the benefits are provided to members of the charitable class for which the charity is supposed to provide benefits.  A hospital may provide free care to low-income people needing healthcare, for example, including free care to a qualified member of the board.  A college may provide scholarships to students needing assistance.  In this case, if the organization was granted exemption to assist young people in start-up businesses, or to assist economic development generally, the benefit of the low cost loan to a qualified recipient would be within the charitable purpose and permissible.

On the deduction question, the issue is the same as applicable to a donor to the hospital or the college.  A donor may claim a charitable contribution deduction for gifts given to the charity for its charitable purposes generally or for a specific type of exempt purpose, but may not claim a deduction if the gift is given for the benefit of a specific individual.  Earmarking for this type of loan is fine; earmarking for a loan to a specific teenager would not be considered charitable.