The Public Broadcasting Service turns 40 this year, and today it will give itself a gift that just might make it feel young again.

The company’s new video portal allows online viewers to stream an array of the company’s best known shows over the Web. The new site gathers more than 130 episodes of nearly 20 programs, including marquee fare such as “Frontline,” “Nova” and “Masterpiece Theater.” PBS says thousands of hours of programming should be available to users by the summer.

In giving its shows away online, PBS is joining on-demand video sites such as Hulu.com and YouTube — places for younger consumers who aren’t wedded to watching TV on a television. Those two sites, both commercial, have been positioning themselves as advertiser-friendly viewing destinations, where video surfers can go to browse among a variety of familiar titles.

PBS’s initial selection of about 20 programs is small compared with the pulpy catalogs maintained by the other sites, but full-length episodes of nearly every show in PBS’ primetime lineup have been put online.