Well Micaela, as far as age is concerned I doubt you'll have a problem with that. Nursing programs here in the US admit a considerable number of second career applicants well over 40!
They dont really discriminate based on age.
As for grades, most programs usually base admissions with prerequisites GPAs or college GPAs (so check the programs for their policy on this) and choose accordingly. You generally get admitted as a pre-nursing undergrad, have the opportunity to take your prerequisites, and would later get to apply for the nursing major once you have completed the prerequisites. Some schools also have criteria that dont solely rely on your GPA but also on pre-admission exams such as the NETs, or the NLN.
Finally, when choosing whether or not you should take the LPN route first, it depends on your situation. For instance, taking the LPN route would probably be faster as opposed to an ASN or a BSN. But if you're really committed to getting an ASN then it would be good idea to explore this route. Just keep in mind to keep your grades really high. As you might have guessed, programs here in the US are extremely competitive!
BEst of luck Micaela!