edit: upon investigation it would appear that Summly Limited has his mother as a director ("MS DIANA JULIA D'ALOISIO") and he is listed as secretary ("NICHOLAS D'ALOISIO-MONTILLA") -- search for "summly" via http://wck2.companieshouse.gov.uk and then select the option to order information.
edit edit: so from the filing the following is true:
Summly Limited has Nick's mother as a director along with the investors, he is a secretary.
The company has 900,000 shares. 600,000 "Ordinary" are allocated to his mother ("DIANA JULIA D'ALOISIO") 120,000 "Series A Preferred" are allocated to GERSHWIN INTERNATIONAL LIMITED and 180,000 "Series A Preferred" are allocated to TWINHILLS LIMITED.
So it seems that Nick has no actual stake in the company and is using his mother as the proxy for his ownership.
edit edit edit: Something might be worth noting, the site lists "Summly Inc" as the company responsible for Summly, not Summly Limited.
Obviously his parents would need to be involved in this. It's possible also that one of the big draws for the investor was the obvious publicity potential (for the venture) of funding someone 15 yrs old vs. someone who is (ho hum) 20. This type of angle plays well in the media.